Add :has selector

[jbhoot]

Pretty much what the title says.

I have added a few test cases all of which pass with flying colours.

Let me know if anything needs to be done.

One note though: make test didn't seem to run my changes. I tested that by adding a bad variable name, but the command did not err out. So I ended up testing with dune exec ./test/performance/test.exe -w after tweaking the file location (google.html) to match the test path.

[jbhoot]

Please let me know if more needs to be done.