Fix tests warnings

Changelog

• Fixes some test and build warnings/errors
• Adds a default test

Custom default filename

Changelog

• Makes the default critical CSS filename configurable via a plugin option. Props @jkphl.

README fixes

Changelog

• Fixes some incorrect code in the README examples.

Version 2: atRules, static typing, better tests

Changelog

• Custom @critical atRule support
• Static typing with flowtype
• Code base modularization & refactor
• More tests

Version 2: atRules, static typing, better tests

• Custom@critical at rule support
• Static typing with flowtype
• Code base modularization & refactor
• More tests