html: handle breakout tags in foreign content.
Also recognize that, in the latest version of the HTML5 spec,
foreign content is not an insertion mode, but a separate concern.
Pass tests10.dat, test 13:
<!DOCTYPE html><body><table><caption><svg><g>foo</g><g>bar</g><p>baz</table><p>quux
| <!DOCTYPE html>
| <html>
| <head>
| <body>
| <table>
| <caption>
| <svg svg>
| <svg g>
| "foo"
| <svg g>
| "bar"
| <p>
| "baz"
| <p>
| "quux"
Also pass tests through test 15:
<!DOCTYPE html><body><table><colgroup><svg><g>foo</g><g>bar</g><p>baz</table><p>quux
R=andybalholm
CC=golang-dev
https://golang.org/cl/
5494078