Support relaxed naming requirements in partial mustaches

So, due to the recursive nature and fixed entry point in parsing, getting options down to the reference parser is a bit tricky. I ended up adding a flag on the parser itself and only setting it for parsing the first expression within a partial mustache, where it's ok to assume you're not trying to do math. This also makes globals and keywords fair game for partial names.

I originally wanted to add a step to try pulling a relaxed name for partials first, but it got hairy with expressions and contexts. I also thought of adding a way to pass options down the parser hierarchy from `readExpression`, but that seemed like a lot of overhead for this one little thing.

Fixes #1165