Add fzf#vim#with_preview function for previewing search result

Close #225

README.md

@@ -152,18 +152,40 @@ let g:fzf_tags_command = 'ctags -R'
 let g:fzf_commands_expect = 'alt-enter,ctrl-x'
 ```
 
-#### Advanced customization using autoload functions
+#### Advanced customization
 
 You can use autoload functions to define your own commands.
 
 ```vim
-" git grep
+" Command for git grep
+" - fzf#vim#grep(command, with_column, [options], [fullscreen])
 command! -bang -nargs=* GGrep
   \ call fzf#vim#grep('git grep --line-number '.shellescape(<q-args>), 0, <bang>0)
 
 " We use VimEnter event so that the code is run after fzf.vim is loaded
-autocmd VimEnter * command! Colors
-  \ call fzf#vim#colors({'left': '15%', 'options': '--reverse --margin 30%,0'})
+autocmd VimEnter * command! -bang Colors
+  \ call fzf#vim#colors({'left': '15%', 'options': '--reverse --margin 30%,0'}, <bang>0)
+
+" Augmenting Ag command using fzf#vim#with_preview function
+"   * fzf#vim#with_preview([[options], preview window, [toggle keys...]])
+"   * Preview script requires Ruby
+"   * Install Highlight or CodeRay to enable syntax highlighting
+"
+"   :Ag  - Start fzf with hidden preview window that can be enabled with "?" key
+"   :Ag! - Start fzf in fullscreen and display the preview window above
+autocmd VimEnter * command! -bang -nargs=* Ag
+  \ call fzf#vim#ag(<q-args>,
+  \                 <bang>0 ? fzf#vim#with_preview('up:60%')
+  \                         : fzf#vim#with_preview('right:50%:hidden', '?'),
+  \                 <bang>0)
+
+" Similarly, we can apply it to fzf#vim#grep. To use ripgrep instead of ag:
+command! -bang -nargs=* Rg
+  \ call fzf#vim#grep(
+  \   'rg --column --line-number --no-heading --color=always '.shellescape(<q-args>), 1,
+  \   <bang>0 ? fzf#vim#with_preview('up:60%')
+  \           : fzf#vim#with_preview('right:50%:hidden', '?'),
+  \   <bang>0)
 ```
 
 Mappings