Add content suffix support (RFC6839)

[nvllsvm]

This adds support for parsing the content suffix from a subtype. A number of them are defined in RFC6839.

For example, application/vnd.test+json will be parsed to the following:

Type: application
Subtype: vnd.test
Suffix: json

[coveralls]

Coverage decreased (-0.1%) to 99.217% when pulling 7d9e913 on nvllsvm:suffix into 532071c on dave-shawley:master.

[dave-shawley]

@nvllsvm +1 ... add yourself to AUTHORS.txt and I'll happily merge. I'll update the userland docs and release it shortly. Thanks for the contribution!

Actually ... I can do the updates. I'll have a new version up shortly. Are you going to add support to https://github.com/sprockets/sprockets.mixins.media_type as well? (nudge)(nudge)

[nvllsvm]

@dave-shawley Thanks for the merge. I'll be taking care of that module along with https://github.com/sprockets/sprockets.mixins.http