Add Helptags command

If you use pathogen, you may notice that the name of the new command
conflicts with the command of the same name from it, but you can still
`call pathogen#helptags()`.

doc/fzf-vim.txt

@@ -70,6 +70,7 @@ COMMANDS                                                      *fzf-vim-commands*
   `History`         |  `v:oldfiles`  and open buffers
   `Snippets`        | Snippets ({UltiSnips}{6})
   `Commands`        | User-defined commands
+  `Helptags`        | Help tags
  -----------------+---------------------------------------------------------------------
 
  - Most commands support CTRL-T / CTRL-X / CTRL-V key bindings to open in a new