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
The `n` (in `namespace`) *is* the first char, see the raw file: <https://raw.githubusercontent.com/dshafik/commonmark/markua/src/CommonMarkConverter.php>