Add a TAP compatible output option.

TAP is the Test Anything Protocol [1], which can be used by automake to
print out more detailed testing results. Also see the specification [2].

[1] http://testanything.org/wiki/index.php/Main_Page
[2] http://podwiki.hexten.net/TAP/TAP.html?page=TAP
Specify TODO tests in TAP output.
Print shortened test names for TAP results.

Instead of "<basedir>/path/to/spec/input.scss", print out
"path/to/spec" or just "<basedir>" if there's no subdirectories.
Use stderr from the tests in their error message.
Output SKIP if no tests were found.
Update ignore file.
Output spec directory for skips.