Initial Markua support

- Add support for `A>` aside syntax
- Add support for multiple environments
- Refactor CommonMark environment into `League\CommonMark\Environment\CommonMark`
- Rename SpecTest to CommonMarkSpecTest
- Start adding Markua spec/tests in tests/specs/markua
also, there shouldn't be a space here - not your fault. The `n` should be the first char on that line, not ` `.