Merge: Vim omnifunc for Nit

Alternative autocompletion for Nit. This one is triggered by a call to
the omnitfunc, usually `ctrl-x ctrl-o`. It will show context
appropriated modules, classes and properties. It shows all properties,
but it does not use static types to show the methods available on the
specific receiver. It also displays some doc!

It uses `nitpick` to generate metadata files analyzed by the Vim plugin.
So in combination with `syntastic`, each time you save, and the program
is valid, it updates the metadata files.

Pull-Request: #1146
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>