Make it possible to disable use of Lua Interface for Vim

Relevant for issue #20:
  #20

Author: xolox

README.md

@@ -66,6 +66,10 @@ You can manually check the globals using the `:CheckGlobals` command.
 
 The name or path of the Lua interpreter used to evaluate Lua scripts used by the plug-in (for example the script that checks for undefined global variables, see `:LuaCheckGlobals`).
 
+### The `lua_internal` option
+
+If you're running a version of Vim that supports the Lua Interface for Vim (see [if_lua.txt][if_lua.txt]) then all Lua code evaluated by the Lua file type plug-in is evaluated using the Lua Interface for Vim. If the Lua Interface for Vim is not available the plug-in falls back to using an external Lua interpreter. You can set this to false (0) to force the plug-in to use an external Lua interpreter.
+
 ### The `lua_compiler_name` option
 
 The name or path of the Lua compiler used to check for syntax errors (defaults to `luac`). You can set this option to run the Lua compiler from a non-standard location or to run a dedicated syntax checker like [lualint][ll].
@@ -165,6 +169,7 @@ This software is licensed under the [MIT license](http://en.wikipedia.org/wiki/M
 [gf]: http://vimdoc.sourceforge.net/htmldoc/editing.html#gf
 [github-lua-ftplugin]: http://github.com/xolox/vim-lua-ftplugin
 [github-misc]: http://github.com/xolox/vim-misc
+[if_lua.txt]: http://vimdoc.sourceforge.net/htmldoc/if_lua.html#if_lua.txt
 [inc]: http://vimdoc.sourceforge.net/htmldoc/options.html#%27include%27
 [inex]: http://vimdoc.sourceforge.net/htmldoc/options.html#%27includeexpr%27
 [ll]: http://lua-users.org/wiki/LuaLint

autoload/xolox/lua.vim

@@ -3,7 +3,7 @@
 " Last Change: June 17, 2014
 " URL: http://peterodding.com/code/vim/lua-ftplugin
 
-let g:xolox#lua#version = '0.7.20'
+let g:xolox#lua#version = '0.7.21'
 let s:miscdir = expand('<sfile>:p:h:h:h') . '/misc/lua-ftplugin'
 let s:omnicomplete_script = s:miscdir . '/omnicomplete.lua'
 let s:globals_script = s:miscdir . '/globals.lua'
@@ -25,7 +25,7 @@ endfunction
 
 function! xolox#lua#getsearchpath(envvar, luavar) " {{{1
   let path = ''
-  if has('lua')
+  if xolox#misc#option#get('lua_internal', has('lua'))
     " Try to get the search path using the Lua Interface for Vim.
     try
       redir => path
@@ -478,7 +478,7 @@ function! xolox#lua#tweakoptions() " {{{1
 endfunction
 
 function! xolox#lua#dofile(pathname, arguments) " {{{1
-  if has('lua')
+  if xolox#misc#option#get('lua_internal', has('lua'))
     " Use the Lua Interface for Vim.
     call xolox#misc#msg#debug("lua.vim %s: Running '%s' using Lua Interface for Vim ..", g:xolox#lua#version, a:pathname)
     redir => output

doc/ft_lua.txt

@@ -10,18 +10,19 @@ Contents ~
   2. The |lua_check_syntax| option
   3. The |lua_check_globals| option
   4. The |lua_interpreter_path| option
-  5. The |lua_compiler_name| option
-  6. The |lua_compiler_args| option
-  7. The |lua_error_format| option
-  8. The |lua_complete_keywords| option
-  9. The |lua_complete_globals| option
-  10. The |lua_complete_library| option
-  11. The |lua_complete_dynamic| option
-  12. The |lua_complete_omni| option
-  13. The |lua_omni_blacklist| option
-  14. The |lua_define_completefunc| option
-  15. The |lua_define_omnifunc| option
-  16. The |lua_define_completion_mappings| option
+  5. The |lua_internal| option
+  6. The |lua_compiler_name| option
+  7. The |lua_compiler_args| option
+  8. The |lua_error_format| option
+  9. The |lua_complete_keywords| option
+  10. The |lua_complete_globals| option
+  11. The |lua_complete_library| option
+  12. The |lua_complete_dynamic| option
+  13. The |lua_complete_omni| option
+  14. The |lua_omni_blacklist| option
+  15. The |lua_define_completefunc| option
+  16. The |lua_define_omnifunc| option
+  17. The |lua_define_completion_mappings| option
  4. Commands                                                  |ft_lua-commands|
   1. The |:LuaCheckSyntax| command
   2. The |:LuaCheckGlobals| command
@@ -150,6 +151,15 @@ The name or path of the Lua interpreter used to evaluate Lua scripts used by
 the plug-in (for example the script that checks for undefined global variables,
 see |:LuaCheckGlobals|).
 
+-------------------------------------------------------------------------------
+The *lua_internal* option
+
+If you're running a version of Vim that supports the Lua Interface for Vim (see
+|if_lua.txt|) then all Lua code evaluated by the Lua file type plug-in is
+evaluated using the Lua Interface for Vim. If the Lua Interface for Vim is not
+available the plug-in falls back to using an external Lua interpreter. You can
+set this to false (0) to force the plug-in to use an external Lua interpreter.
+
 -------------------------------------------------------------------------------
 The *lua_compiler_name* option