Merge: Vim plugin fixes and make Nitdoc a command with search capabilities

You can now use Nitdoc as a command in Vim.

* Search the doc for the word under the cursor with `:Nitdoc`
* Display the doc of the `String` class (followed by all docs using the word "String", case-insensitive) with `:Nitdoc String`
* Display the doc of `Int::%` with `:Nitdoc modulo`
* Search for a string of words (in the given order) with `:Nitdoc not null`

Some query can produce quite a lot results. `:Nitdoc the` will fill the preview window with 33 379 lines of doc, but there no noticeable delay.

### Bonus tip!

Navigate in the doc by moving the cursor above a word in the preview window and using ctrl-D (or `:Nitdoc`), exactly as you would do in Nit code.

Pull-Request: #1256
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>