Bump to 0.31.1

0.30.2-1 release.

Bump revision to 5.

Revision 4 release.

Update to 0.29.0.

Note that the default setting for 'safe' is now false.
The command line program now takes '--unsafe' instead of '--safe'.

Bump to 0.28.3.

Update tests.

Require cmark 0.26.0.

Update spec tests.

Fixed link for filters dir.