Add option cacheFile

Closes #101.

I did some more testing and it seems unfortunately that if you run your server using `nodemon` and then press `Ctrl+C`, I can't find a way to intercept the event and write out to `cacheFile`. Using `grunt-nodemon` seems to work just fine though. Also, since I am trying to intercept `SIGINT` but still need to be courteous and allow other user-defined `SIGINT` handlers to run, it is only possible to intercept `SIGINT` once. If someone registers a `SIGINT` handler that doesn't actually kill the server, subsequent `SIGINT`s won't result in the cache file being updated.
Was this a bug in the current library where we use `cssPath` and `lessPath` mixed?