Merge: vim plugin: intro NitExecute to interpret the current file with `nit`

Merge: vim plugin: intro NitExecute to interpret the current file with `nit`

Adds the `:NitExecute` command (or `:NitE` as shortcut) to interpret the current file with `nit`.

This is very similar to the manual `:!nit %` but it saves modified buffers to a temp file to execute the content of the current buffer and not the last saved content.

This is completely up to you but I recommend mapping this to `ctrl-f` to fit between `ctrl-d -> Nitdoc` and `ctrl-g -> NitGitGrep`. Here are the required lines for `.vimrc`:

~~~
" Map the Nitdoc command to Ctrl-D
map <C-d> :Nitdoc<enter>

" Map the NitGitGrep function to Ctrl-G
map <C-g> :call NitGitGrep()<enter>

" Map the NitExecute function to Ctrl-F <------- The new one
map <C-f> :NitExecute<enter>
~~~

Pull-Request: #1717
Reviewed-by: Jean Privat <jean@pryen.org>