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()`.

README.md

@@ -45,6 +45,7 @@ Commands
 | `History`        | `v:oldfiles` and open buffers                                             |
 | `Snippets`       | Snippets ([UltiSnips][us])                                                |
 | `Commands`       | User-defined commands                                                     |
+| `Helptags`       | Help tags                                                                 |
 
 - Most commands support `CTRL-T` / `CTRL-X` / `CTRL-V` key
   bindings to open in a new tab, a new split, or in a new vertical split.

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

plugin/fzf.vim

@@ -472,6 +472,24 @@ endfunction
 
 command! -bang Marks call s:marks(<bang>0)
 
+" ------------------------------------------------------------------
+" Help tags
+" ------------------------------------------------------------------
+function! s:helptag_sink(line)
+  execute 'help' split(a:line)[0]
+endfunction
+
+function! s:helptags(bang)
+  let tags = split(globpath(&runtimepath, '**/doc/tags'), '\n')
+
+  call s:fzf({
+  \ 'source':  'cat '.join(map(tags, 'shellescape(v:val)'))."| sort | awk '{printf \"%-40s%s\\n\", $1, $2}'",
+  \ 'sink':    function('s:helptag_sink'),
+  \ 'options': '+m --tiebreak=begin'}, a:bang)
+endfunction
+
+command! -bang Helptags call s:helptags(<bang>0)
+
 " ----------------------------------------------------------------------------
 " Completion helper
 " ----------------------------------------------------------------------------