Code Splitting

[chriseppstein]

Once OptiCSS runs on an analysis, the styles can be split by using subsets of the analysis to produce a base stylesheet for styles shared across code splits or styles that weren't enumerated as a part of a code split. Styles specific to each code split will be extracted to a split css file.

TBD: rewrite data may still be centralized, or it might be split-able as well.

Note: With declaration merging enabled, the benefits of code-splitting may not result in enough savings to warrant the extra complexity.