Tags whitelist support

[mloginov]

options: {
sanitize: true,
allowedTags: 'iframe|img|video'
}

For #133

[chjj]

Please refactor this so a RegExp is not dynamically created each call. A hash table check would be faster than a regex: reduce an array of tags to an object and check the object's property.

[paibamboo]

please continue :)

[joshbruce]

I know this is old, see #956

Closing:

1. Never refactored.
2. Conflicts.
3. Missing tests.
4. Stale.
5. Added whitelist tag support to work alongside sanitizer #710