Update Stylus to ~0.48.1

This version of Stylus includes https://github.com/LearnBoost/stylus/pull/1653 which resolves issue #2
Correct variable name in README usage example
Add Stylus as peer dependency

Sometimes a specific Stylus version is desired that is different than the one bundled with this plugin.
Merge pull request #5 from jas/readme-usage

Correct variable name in README usage example
Merge pull request #4 from jas/stylus-0.48

Update Stylus to ~0.48.1
Merge pull request #7 from jas/stylus-peer-dep

Add Stylus as peer dependency
add .gitignore
bump package version
Correctly use custom Stylus version if available

Per the README (https://github.com/gabrielgrant/broccoli-stylus-single#stylus-version) the Stylus version used by this plugin should be configurable.

`peerDependencies` doesn't appear to work in the way that I had expected it to.

This change will check for `stylus` in the root project and fall back to the version included as a dependency of `broccoli-stylus-single` if it's not present.
Merge pull request #9 from jas/fix-custom-stylus

Correctly use custom Stylus version if available