Improved plugins config table handling (#1356)

* Warns user if trying to disable a plugin that is already
  enabled when doing `config.plugins.plugin_name = false` and also
  prevents replacing the current plugin config table with the false
  value to mitigate runtime issues.
* Uses a merge strategy by default when assigning a table to a plugin
  config to prevent a user from removing a plugin default config values
  as experienced and explained on this issue lite-xl-plugins#158
* This change is basically backwards compatible, but will require a
  change on the settings ui plugin on how it checks for already
  enabled plugins, since rawget will no longer be a working hack
  or workaround for this.
* As suggested by Adam dropped loaded key and switched to package.loaded

data/core/config.lua

@@ -1,3 +1,5 @@
+local common = require "core.common"
+
 local config = {}
 
 config.fps = 60
@@ -55,13 +57,47 @@ config.max_clicks = 3
 -- set as true to be able to test non supported plugins
 config.skip_plugins_version = false
 
+-- holds the plugins real config table
+local plugins_config = {}
+
+-- virtual representation of plugins config table
 config.plugins = {}
--- Allow you to set plugin configs even if we haven't seen the plugin before.
+
+-- allows virtual access to the plugins config table
 setmetatable(config.plugins, {
-  __index = function(t, k)
-    local v = rawget(t, k)
-    if v == true or v == nil then v = {} rawset(t, k, v) end
-    return v
+  __index = function(_, k)
+    if not plugins_config[k] then
+      plugins_config[k] = { enabled = true, config = {} }
+    end
+    if plugins_config[k].enabled ~= false then
+      return plugins_config[k].config
+    end
+    return false
+  end,
+  __newindex = function(_, k, v)
+    if not plugins_config[k] then
+      plugins_config[k] = { enabled = nil, config = {} }
+    end
+    if v == false and package.loaded["plugins."..k] then
+      local core = require "core"
+      core.warn("[%s] is already enabled, restart the editor for the change to take effect", k)
+      return
+    elseif plugins_config[k].enabled == false and v ~= false then
+      plugins_config[k].enabled = true
+    end
+    if v == false then
+      plugins_config[k].enabled = false
+    elseif type(v) == "table" then
+      plugins_config[k].enabled = true
+      plugins_config[k].config = common.merge(plugins_config[k].config, v)
+    end
+  end,
+  __pairs = function()
+    return coroutine.wrap(function()
+      for name, status in pairs(plugins_config) do
+        coroutine.yield(name, status.config)
+      end
+    end)
   end
 })