Add :FernDo command and deprecate :FernFocus command

[lambdalisue]

To solve #94, :FernDo command has added.

For example, if users want to close fern drawer after openaction then

nmap <buffer><silent> <Plug>(fern-my-open-and-close)
      \ <Plug>(fern-action-open)
      \ :<C-u>FernDo close -drawer -stay<CR>

In favor of :FernDo, the :FernFocus command become deprecated. Use :FernDo : instead.

[lambdalisue]

NOTE:

It seems the above example mapping does not work...