Rename ctags.options to ctags_recursive.options

According to https://github.com/ludovicchabant/vim-gutentags/issues/33,
ctags' --recursive option was moved to an option file to allow it to be
overridden by user options files. This change makes it clear what the
purpose of this options file is.

autoload/gutentags/ctags.vim

@@ -80,9 +80,9 @@ function! gutentags#ctags#generate(proj_dir, tags_file, write_mode) abort
             endif
             let l:cmd .= ' -s "' . l:cur_file_path . '"'
         endif
-        " Pass the Gutentags options file first, and then the project specific
-        " one, so that users can override the default behaviour.
-        let l:cmd .= ' -o "' . gutentags#get_res_file('ctags.options') . '"'
+        " Pass the Gutentags recursive options file before the project
+        " options file, so that users can override --recursive.
+        let l:cmd .= ' -o "' . gutentags#get_res_file('ctags_recursive.options') . '"'
         let l:proj_options_file = a:proj_dir . '/' .
                     \g:gutentags_ctags_options_file
         if filereadable(l:proj_options_file)