Various minor efficiency tweaks to improve perf on large documents.

In my tests parsing the 7-megabyte single page WHATWG HTML spec, these
tweaks resulted in a 1.3x performance improvement.
Deep freeze the built-in configs.

Based on PR #75 from @pda.

This also adds Sanitize::Config.merge(), which can be used to safely
deep-merge two configs (details in the readme).
Include HISTORY.md in the docs.