Switch to a multipass process

[adamgian]

Currently running as a single pass process where selectors are identified and replaced on the fly.

Process can be changed to a multipass along the lines of:

1. Identify selectors in files first, logging all found selectors
2. Generate replacement selectors
3. Identify selectors in files again, this time replacing with minified selectors

[adamgian]

This will allow the following current issues (and future ones) to be realized:

• Option to run minify-selectors in parallel (#14)
• Order selectors based on frequency, so that they are assigned the shorter generated selectors first (#29)

[adamgian]

As of 4b7e206eeef0d2995e8cfdc52689e96d7fc4e8cf multi-step process is in place