GFM report format

markdown formatting tweaks, github file links, and test

support subpath in repo

github url support in HTML output

use to_markdown in format call

update non ruby1.9 line count

add markdown to valid format list

escape file names

move repo validation and parsing to Brakeman#set_options
Merge remote-tracking branch 'github/gfm-output-upstream'

GitHub flavored markdown output