Implement limits for maximum number of symbols in autocomplete

Implemented to avoid excessive memory usage when loading big files with
many unique words.

data/core/config.lua

@@ -16,5 +16,6 @@ config.line_height = 1.2
 config.indent_size = 2
 config.tab_type = "soft"
 config.line_limit = 80
+config.max_symbols = 2000
 
 return config

data/plugins/autocomplete.lua

@@ -25,17 +25,29 @@ function autocomplete.add(t)
   autocomplete.map[t.name] =  { files = t.files or ".*", items = items }
 end
 
+local max_symbols = config.max_symbols or 2000
 
 core.add_thread(function()
   local cache = setmetatable({}, { __mode = "k" })
 
   local function get_symbols(doc)
+    if doc.disable_symbols then return {} end
     local i = 1
     local s = {}
+    local symbols_count = 0
     while i < #doc.lines do
       for sym in doc.lines[i]:gmatch(config.symbol_pattern) do
+        if not s[sym] then
+          symbols_count = symbols_count + 1
+          if symbols_count > max_symbols then
+            s = nil
+            doc.disable_symbols = true
+            collectgarbage('collect')
+            return {}
+          end
           s[sym] = true
         end
+      end
       i = i + 1
       if i % 100 == 0 then coroutine.yield() end
     end