HTML tag whitelisting & attribute blacklisting

[grugnog]

This should be configurable through the Mangle struct, with sensible defaults.

Currently text inside every tag is mangled, however some tags such as script should normally be skipped via a whitelist approach.

Conversely, right now no attributes are mangled, however specific attributes should be via a blacklist approach - for example alt and title. Could be "tag wide", or "tag specific" (img.alt) configuration - need to look at spec and decide if a broad approach could work.