feat(core): add SPC h g/G to run helpgrep asynchronously

Problem: `:helpgrep` is too slow, there is no key binding for
         asynchronously helpgrep.
Solution: - Add `SPC h g` to run helpgrep asynchronously.
          - Add `SPC h G` to run helpgrep with cursor word
            asynchronously

autoload/SpaceVim/layers/core.vim

@@ -221,6 +221,26 @@ function! SpaceVim#layers#core#config() abort
           \ ]
           \ ]
           \ , 1)
+  let lnum = expand('<slnum>') + s:lnum - 1
+  call SpaceVim#mapping#space#def('nnoremap', ['h', 'g'], 'call SpaceVim#plugins#helpgrep#help()',
+        \ ['asynchronous-helpgrep',
+        \ [
+          \ '[SPC h g] is to run helpgrep asynchronously',
+          \ '',
+          \ 'Definition: ' . s:filename . ':' . lnum,
+          \ ]
+          \ ]
+          \ , 1)
+  let lnum = expand('<slnum>') + s:lnum - 1
+  call SpaceVim#mapping#space#def('nnoremap', ['h', 'G'], 'call SpaceVim#plugins#helpgrep#help(expand("<cword>"))',
+        \ ['asynchronous-helpgrep-with-cword',
+        \ [
+          \ '[SPC h g] is to run helpgrep asynchronously with cword',
+          \ '',
+          \ 'Definition: ' . s:filename . ':' . lnum,
+          \ ]
+          \ ]
+          \ , 1)
   call SpaceVim#mapping#space#def('nnoremap', ['b', 'd'], 'call SpaceVim#mapping#close_current_buffer()', 'delete-this-buffer', 1)
   call SpaceVim#mapping#space#def('nnoremap', ['b', 'D'],
         \ 'call SpaceVim#mapping#kill_visible_buffer_choosewin()',

autoload/SpaceVim/plugins/helpgrep.vim

@@ -0,0 +1,27 @@
+"=============================================================================
+" helpgrep.vim --- asynchronous helpgrep
+" Copyright (c) 2016-2019 Wang Shidong & Contributors
+" Author: Wang Shidong < wsdjeg@outlook.com >
+" URL: https://spacevim.org
+" License: GPLv3
+"=============================================================================
+
+function! s:generate() abort
+    return filter(split(&rtp, ','), 'isdirectory(v:val)')
+endfunction
+
+function! s:help_files() abort
+    return globpath(&rtp, 'doc/*.txt', 0, 1)
+endfunction
+
+function! SpaceVim#plugins#helpgrep#help(...) abort
+  call SpaceVim#plugins#flygrep#open({
+          \ 'input' : get(a:000, 0, ''),
+          \ 'files' : s:help_files(),
+          \ })
+endfunction
+
+command! GrepRtp call SpaceVim#plugins#flygrep#open({
+          \ 'input' : '',
+          \ 'dir' : s:generate(),
+          \ })

docs/cn/documentation.md

@@ -1693,6 +1693,8 @@ Denite/Unite 是一个强大的信息筛选浏览器，这类似于 Emacs 中的
 | ----------- | -------------------------------------------------- |
 | `SPC h SPC` | 使用 fuzzy find 模块展示 SpaceVim 帮助文档章节目录 |
 | `SPC h i`   | 获取光标下单词的帮助信息                           |
+| `SPC h g`   | 异步执行`:helpgrep`                                |
+| `SPC h G`   | 异步执行`:helpgrep`，并搜索光标下的词              |
 | `SPC h k`   | 使用快捷键导航，展示 SpaceVim 所支持的前缀键       |
 | `SPC h m`   | 使用 Unite 浏览所有 man 文档                       |
 

docs/documentation.md

@@ -1694,6 +1694,8 @@ getting help info about functions, variables etc:
 | ------------ | ----------------------------------------------------------------------------- |
 | `SPC h SPC`  | discover SpaceVim documentation, layers and packages using fuzzy finder layer |
 | `SPC h i`    | get help with the symbol at point                                             |
+| `SPC h g`    | run `:helpgrep` asynchronously                                                |
+| `SPC h G`    | run `:helpgrep` asynchronously with the word under cursor                     |
 | `SPC h k`    | show top-level bindings with which-key                                        |
 | `SPC h m`    | search available man pages                                                    |