Implement Selective for a11y testing

[apetro]

https://github.com/ChristianMurphy/selective

[ChristianMurphy]

Selective 0.1.0 is available as for CI/CLI as a rehype plugin and as an in IDE experience for atom through an atom linter plugin.
It is 100% complete in terms of selector support for detecting potential issues.

There will be some additional polish work, supporting shareable configs, better distinguishing between different log levels, and allowing rules to be disabled from inline comments that is planned before I'll declare the tool "stable 1.0".

Further down the road I'm looking at adding VSCode and WebStorm integrations.