Added in performance tests and internal cache for file data

As mentioned in #3, we started running into performance issues. This PR introduces a cache in front of `fs` to allow for faster reads. In this PR:

- Added internal cache in front of `fs.readFile`
- Added performance benchmarking
- Added test to verify cache stays sanitized
- Takes down a 3 second test suite to 1 second

Fixes #3.

/cc @mlmorg @Raynos