Add option to specify a type for Highlighter serialization

[jonbrooks]

Optionally allow clients to specify a type for highlighter serialization different from the highlighter's type. In this way, clients can use the more responsive textContent type for creating highlights, but serialize the highlights into a TextRange format for better consistency across browsers.

Code play the same game done in deserialize that converts from one type to another when necessary.

If this doesn't fit with your roadmap, feel free to ignore, but I thought this might be of use to others. If there is a good reason why this is a bad idea, I'd love to hear it.

[timdown]

Yep, I like this. Thanks very much.

[jonbrooks]

Great. Glad you liked it.