Perf issue: `criteria` called twice per file

[springmeyer]

Noticing that for one file and one processor the processors criteria method is called twice. Because the criteria method may need to open a file, and some files are big, a speedup could be had by only doing this work once.

It looks like the double call is happening because applicable calls criteria and applicable is called in two places:

• https://github.com/mapbox/preprocessorcerer/blob/master/preprocessors/index.js#L50
• https://github.com/mapbox/preprocessorcerer/blob/master/preprocessors/index.js#L70