Live HTML initial implementation (off by default)

This merges the ResearchLiveHTML branch, which implements updating the browser in Live Preview as you type in an HTML file. Currently, the main LiveHTML features are turned off since it's not quite ready yet, but can be enabled by setting `brackets.livehtml = true` in devtools.

The existing live highlighting and reload-on-save functionality for HTML should continue to work, although we're now using a new tokenizer/parser in order to generate the highlight ranges.