Spec compliance testing

[thekid]

This pull request adds the infrastructure for running the CommonMark spec tests.

$ curl -sSL https://github.com/commonmark/commonmark-spec/raw/master/spec.txt > spec.txt
# ...

$ xp test src/it/php/ --spec=spec.txt
# ...

Test cases:  121 succeeded, 530 failed
Memory used: 5783.13 kB (7357.30 kB peak)
Time taken:  0.096 seconds (0.411 seconds overall)

Lots of TODOs 🙃

[thekid]

#17 is merged, fixing two tests

[thekid]

After some small adjustments we now have:

Test cases:  126 succeeded, 525 failed
Memory used: 5780.15 kB (7324.12 kB peak)
Time taken:  0.074 seconds (0.385 seconds overall)

Scanning through these lots of them seem to be related to:

• Whitespace handling and emitting (like <ul>\n<li> vs. <ul><li>)
• HTML support (we don't have it!)
• URL encoding
• Support for explicit links (via <link>)
• Escaping via backslashes

[thekid]

Will merge this (but not add it to the test suite) in order to iteratively reach more spec compliance