Check `gulp-flowtype` code with jshint and flow

I tried to use `gulp-flowtype` and suddenly I needed to specify the path to the declarations: `--libs` option. http://flowtype.org/docs/declarations.html#
Also, I noticed that codestyle is a bit inconsistent, so I added `npm run lint` command which also run `flow` on `index.js` to see that happens:)

Currently, it looks impossible to pass every `flow` check, because of https://github.com/facebook/flow/issues/73 and some necessary type annotation.