Add Standard Markdown testsuite.

You could run the Parsedown testsuite only with:
    phpunit --testsuite ParsedownTests

And you could run the Standard Markdown one with:
    phpunit --testsuite StandardMarkdown

See more at http://standardmarkdown.com/
Add Docblock and attribution to JS test runner
Merge pull request #217 from hkdobrev/standard-markdown

Standard markdown