code blocks: use textContent, enable gfm_code by default, add & enable trim_code
except <body>, always nest target in generic wrapper. #42
trim global leading/trailing newlines
fix nodes getting plucked from DOM. #42.
Support custom gfm code block delimiters

By setting gfm_code to a string, a delimiter can be chosen.

Issue #22
readme update for gfm_code changes. close #22.