BUG CSSContentParser fails if CLI tidy doesn't output anything.

In cases where CLI tidy fails to recognise HTML with errors or
unknown HTML5 elements, it doesn't output any content and
CSSContentParser fails.

This adds the --force-output parameter so that it always outputs
the input content, even if it wasn't completely fixed by tidy.

It seems that tidy class in PHP already does this, so this just
fixes the CLI use of tidy in CSSContentParser to be consistent.

This fix can be observed on the HHVM travis builds, as tidy isn't
compiled into HHVM as an extension.
Merge pull request #3468 from halkyon/tidy_fixes

BUG CSSContentParser fails if CLI tidy doesn't output anything.