Re-support type="text/traceur" to enable global semantics for scripts

...tics for scripts

	Fixes #822

	The difference between scripts with type="module" and "text/traceur" is that the "text/traceur" script's globals will be global to all the other scripts, just like regular es5 script tags.
Another option is to just do `script[type]` and then check the type in src/WebPageTranscoder.js.