Implement Ruby's <rb> and <rtc>

[nox]

This has to wait for the situation between W3C and WHATWG to settle down:

https://www.w3.org/Bugs/Public/show_bug.cgi?id=26189

All the ignored tests from ruby.dat are related to implicit closing with rb and rtc tags.

[nox]

The two following tests also fail for the same reasons:

• tb: tests19.dat-18
• tb: tests19.dat-21

[nox]

whatwg/html#101