Add more CSS-style selectors

[percyhanna]

• [x] ' ': Descendent selector
• [x] '>': Child selector
• [x] ',': Union selector
• [x] '?=': Extended attribute selectors (e.g. ~=, |=, ^=, $=, *=)

[percyhanna]

WIP in #3

[percyhanna]

Added and released as part of v1.0.0 release.