lexer options

[progrium]

Based on experience trying to get this to co-exist with a templating syntax (as is with Sigil), it would be nice if there was a method that let you expand with more options since we have a decent little lexer. It would be nice to have options like AllowUnbracketed = false and a way to say only expand within or outside {{ and }} tokens.

The thinking is that this allows you to do full unbracketed parsing outside template blocks, and only unambiguous bracketed expansions inside template blocks. This lets you use variables for template blocks, but doesn't get in the way of variable references of the templating syntax.