simplify delimiter specification

This commit allows users to specify delimiters as a pair of start and
stop tags. It constructs the required regular expression taking care
to escape regexes as necessary. It uses escape_regex from utils.R.
add escape_regex to support knit_expand
add missing parenthesis in function definition
modify delimiters in examples
add tests for templates.
Merge pull request #448 from ramnathv/master

Simplify specification of delimiters in knit_expand