Add the plugin for stripping flow types

[xaviervia]

This is WIP just because the eslint-plugin-flowtype works for ignoring the flowtype checks, but doesn't lint the flow related syntax. I'd like to figure out a way of making that work before merging.

[xaviervia]

@pirelenito @Deschtex the situation right now is:

• Setting up babel-eslint as the parser makes standard play nicely with flowtype annotations, but
• …the linter doesn't give any feedback on linting rules for flow type annotations. That is, it's not possible to set style guides for annotations, only for regular ES-whatever code.

I'm not sure this is a dealbreaker. Overall my idea was to add flowtype without causing any issues for people not intending to use it, and that's pretty much solved. I don't know if having linting of flow annotations is important for first drop. I checked out the two projects for adding linting rules for it and it doesn't seem that either of those is mature or bug free.

If you agree that this is not a dealbreaker I'll clean up and remove the WIP mark.

[pirelenito]

I think that should be fine! Amazing work :)

Stupid question about the linting... do we need to make any change on saguijs/eslint-config-sagui ?

[xaviervia]

@pirelenito yes we do. I'll PR that next.