Check duplicates for normal imports and flow type imports separately

[sgtpepper43]

Fix #225

If import is a flow type import, add to separate map for duplicate checking.

[sgtpepper43]

The travis ci build failed because nvm install 5 timed out. Probably just needs to be restarted.

[benmosher]

Thanks! 😄

[dmnd]

Nice, thanks!

[mizchi]

I encountered alike problem on import/named.

import type { Connector } from 'react-redux'

https://github.com/flowtype/flow-typed/blob/master/definitions/npm/react-redux_v5.x.x/flow_v0.30.x-/react-redux_v5.x.x.js#L33-L36

Connector is just a type on definition file but there is no instance.

[fabiomcosta]

:/
Too bad this wasn't introduced with an option to disable this behavior.
You can import types and other things with the same import with:

import defaultNamed, { named, typed namedType } from './module';

This is fine as the default, but it would be nice to provide an option to keep the previous behavior.