Adds plaintext escaping functions.

bleach.escapePlaintextIntoElementContext neutralizes all HTML characters;
bleach.escapePlaintextIntoAttribute renders text safe for inserting into an attribute.
Escape using entity names if available
Merge pull request #5 from mcav/escape-plaintext-bug-899070

 Bug 907805 - [email] Add plaintext escaping functions to bleach.js. r=asuth