set stylus as a peer dependency

[yocontra]

The version of stylus has to keep being updated and there is a constant mismatch between the latest and the version in this module. Currently the version in this module does not support sourcemaps, while the latest does. The second you add nib to a project your sourcemaps go out the window. This could all be solved by using peer dependencies or using a more laid back range for stylus.

[notslang]

that sounds like a good idea, wanna submit a PR?

[eknkc]

+1, solves #301

[optikfluffel]

+1

[adriengibrat]

Seems sytlus is not yet a peer dependency because of #297