automatic prefixing for pseudo selectors (and values)

[fuzetsu]

For example ::placeholder is not supported in Edge and causes a syntax error (flems).

Would be nice if we handled this automatically, and replaced with prefixed versions automatically like we do with css properties.

This might be the time to look into prefixing special css values as well.

[fuzetsu]

Still need to look into vendor prefixing functions within values.