Improve performance by reducing the number of phpcs calls

[sirbrillig]

One of the biggest overheads is calling out to phpcs. We may be able to get a large speed boost by batching how many time we call it.

I have two possible approaches but I'm not sure which is the right one to take:

• Batch phpcs into two invocations: one for new files, one for old #114
• Batch phpcs calls into a single invocation #110