Allow three consecutive hyphens in body, fixes #245.

Parsing the YAML front matter from a post body broke if the post itself contained three consequtive hyphens (e.g., a heading).

Make the regex less greedy to look for the first set of closing hyphens (rather than the last set).
Force initial three hyphens to be the first three characters of the file.

This make the regex more robust, and prevents accidental parsing of non YAML triple hyphen pairs.
Merge #262
Merge branch 'gh-pages' of github.com:prose/prose into gh-pages