[Tags] Add g:fzf_tags_command

Close #138

doc/fzf-vim.txt

@@ -154,9 +154,12 @@ Command-local options~
     " [Buffers] Jump to the existing window if possible
     let g:fzf_buffers_jump = 1
 
-    " [[B]Commits] to customize the options used by 'git log':
+    " [[B]Commits] Customize the options used by 'git log':
     let g:fzf_commits_log_options = \
         '--graph --color=always --format="%C(auto)%h%d %s %C(black)%C(bold)%cr"'
+
+    " [Tags] Command to generate tags file
+    let g:fzf_tags_command = 'ctags -R'
 <
 
 Advanced customization using autoload functions~