Add Caching

[wbyoung]

This addresses issues of concern in #1, and may or may not resolve that issue entirely.

The cache is not quite ideal at the moment since it's not based on the contents of the postcss.config.js file, but this is a good step forward.

[coveralls]

Coverage remained the same at 100.0% when pulling b6ec645 on caching into 3d377ed on master.

[wbyoung]

Oh, it's also worth noting here… the cache is on disk, though it was originally attempted to be made in memory since a server could be long-running (as indicated in #1). Unfortunately, this didn't work with a simple use case, running under nodemon because nodemon kills the process tree which means the server we run is killed off every time nodemon notices the process needs to restart. Arguably, this is a bug in nodemon, but they specifically added this feature, and it would just cause conflicts to suggest that nodemon be run in a specific way in order to use this plugin, so it's probably better to just go with what they have and expect/allow the server to be killed off while keeping a cache around.