Support multiple filetypes

Fix #786

doc/cmp.txt

@@ -603,7 +603,7 @@ How to setup on the specific buffer?~
 
 You can setup buffer specific configuration like this.
 >
-  cmp.setup.filetype('markdown', {
+  cmp.setup.filetype({ 'markdown', 'help' }, {
     sources = {
       { name = 'path' },
       { name = 'buffer' },

lua/cmp/config.lua

@@ -45,11 +45,13 @@ end
 
 ---Set configuration for filetype
 ---@param c cmp.ConfigSchema
----@param filetype string
-config.set_filetype = function(c, filetype)
+---@param filetypes string[]|string
+config.set_filetype = function(c, filetypes)
+  for _, filetype in ipairs(type(filetypes) == 'table' and filetypes or { filetypes }) do
     local revision = (config.filetypes[filetype] or {}).revision or 1
     config.filetypes[filetype] = c or {}
     config.filetypes[filetype].revision = revision + 1
+  end
 end
 
 ---Set configuration for cmdline