v3.1.1

Fixes:

• Support DOM elements of type root in the :root pseudo-selector c4d2b7b

Dependency upgrades:

• bump domutils, domhandler & htmlparser2 (#312) c4d2b7b
• bump nth-check from 1.0.2 to 2.0.0 187c9cd
• bump css-what from 3.4.1 to 4.0.0 4241106