Add setSimpleTag() for basic custom tags (#511)

Still need to add some new docs for this, but this is what I was thinking for the implementation. Let me know if anything is missing or looks wrong. Thanks!
OK, gotcha. I did try that (removing the `parser.on()` statements), but it broke a bunch of my tests. One thing in particular that was happening is string tokens were being passed to the compiler with the quotes (which I'd assume is not the desired behaviour).