Simplify specification of delimiters in knit_expand

This pull request simplifies delimiter specification by allowing users to just specify starting and ending tags as opposed to a regular expression. It has the advantage of being simple and allowing users to customize tags easily without entering regex escape hell.

I have NOT updated documentation and examples, since I wanted to make sure that you were fine with this change in principle. Once you confirm, I can update the examples and tests (if any).

BTW knit_expand was exactly what I was looking for to allow plugins in the blogging engine that I am developing. So thanks for putting this in place!