Improve `drawwhitespace` plugin

Add configuration options to specify:
- characters to substitute and their substitution;
- whether to substitute at the beginning, middle or end of the line;
- the color of the substitution;
- the color for the beginning, middle or end;
- the minimum number of white space to show in the middle.

data/plugins/drawwhitespace.lua

@@ -3,34 +3,108 @@
 local style = require "core.style"
 local DocView = require "core.docview"
 local common = require "core.common"
+local config = require "core.config"
+
+config.plugins.drawwhitespace = common.merge({
+  show_leading = true,
+  show_trailing = true,
+  show_middle = true,
+
+  show_middle_min = 1,
+
+  color = style.syntax.whitespace or style.syntax.comment,
+  leading_color = nil,
+  middle_color = nil,
+  trailing_color = nil,
+
+  substitutions = {
+    {
+      char = " ",
+      sub = "·",
+      -- You can put any of the previous options here too.
+      -- For example:
+      -- show_middle_min = 2,
+      -- show_leading = false,
+    },
+    {
+      char = "\t",
+      sub = "»",
+    },
+  },
+}, config.plugins.drawwhitespace)
+
+local function get_option(substitution, option)
+  if substitution[option] == nil then
+    return config.plugins.drawwhitespace[option]
+  end
+  return substitution[option]
+end
 
 local draw_line_text = DocView.draw_line_text
-
 function DocView:draw_line_text(idx, x, y)
   local font = (self:get_font() or style.syntax_fonts["whitespace"] or style.syntax_fonts["comment"])
-  local color = style.syntax.whitespace or style.syntax.comment
   local ty = y + self:get_line_text_y_offset()
   local tx
   local text, offset, s, e = self.doc.lines[idx], 1
   local x1, _, x2, _ = self:get_content_bounds()
   local _offset = self:get_x_offset_col(idx, x1)
-  offset = _offset
-  while true do
-    s, e = text:find(" +", offset)
-    if not s then break end
-    tx = self:get_col_x_offset(idx, s) + x
-    renderer.draw_text(font, string.rep("·", e - s + 1), tx, ty, color)
-    if tx > x + x2 then break end
-    offset = e + 1
-  end
-  offset = _offset
-  while true do
-    s, e = text:find("\t", offset)
-    if not s then break end
-    tx = self:get_col_x_offset(idx, s) + x
-    renderer.draw_text(font, "»", tx, ty, color)
-    if tx > x + x2 then break end
-    offset = e + 1
+
+  for _, substitution in pairs(config.plugins.drawwhitespace.substitutions) do
+    local char = substitution.char
+    local sub = substitution.sub
+    offset = _offset
+
+    local show_leading = get_option(substitution, "show_leading")
+    local show_middle = get_option(substitution, "show_middle")
+    local show_trailing = get_option(substitution, "show_trailing")
+
+    local show_middle_min = get_option(substitution, "show_middle_min")
+
+    local base_color = get_option(substitution, "color")
+    local leading_color = get_option(substitution, "leading_color") or base_color
+    local middle_color = get_option(substitution, "middle_color") or base_color
+    local trailing_color = get_option(substitution, "trailing_color") or base_color
+
+    local pattern = char.."+"
+    while true do
+      s, e = text:find(pattern, offset)
+      if not s then break end
+
+      tx = self:get_col_x_offset(idx, s) + x
+
+      local color = base_color
+      local draw = false
+
+      if e == #text - 1 then
+        draw = show_trailing
+        color = trailing_color
+      elseif s == 1 then
+        draw = show_leading
+        color = leading_color
+      else
+        draw = show_middle and (e - s + 1 >= show_middle_min)
+        color = middle_color
+      end
+
+      if draw then
+        -- We need to draw tabs one at a time because they might have a
+        -- different size than the substituting character.
+        -- This also applies to any other char if we use non-monospace fonts
+        -- but we ignore this case for now.
+        if char == "\t" then
+          for i = s,e do
+            tx = self:get_col_x_offset(idx, i) + x
+            tx = renderer.draw_text(font, sub, tx, ty, color)
+          end
+        else
+          tx = renderer.draw_text(font, string.rep(sub, e - s + 1), tx, ty, color)
+        end
+      end
+
+      if tx > x + x2 then break end
+      offset = e + 1
+    end
   end
+
   draw_line_text(self, idx, x, y)
 end