html: Remove SKIP_{STYLE,IMAGES,LINKS} flags (see #62)

Even with these flags active, injections and XSS are still easily possible.

Providing full attribute checking, HTML validation, ... is out of the
scope of Hoedown, therefore this "security" features only create
a false sense of security rather than actually providing it.
Revert .gitignore

This reverts commit b751c1708950292e96bd92e803a0db36b74f0b6b.