This completely covers the tags used by exp/html's parser.

Before:
295 atoms; 1406 string bytes + 2048 tables = 3454 total data
BenchmarkLookup    50000         59841 ns/op

After:
322 atoms; 1508 string bytes + 2048 tables = 3556 total data
BenchmarkLookup    50000         60159 ns/op

R=r
CC=golang-dev
https://golang.org/cl/6296045