Added Text.Pandoc.Compat.Monoid.

This allows pandoc to compile with base < 4.5, where Data.Monoid
doesn't export `<>`.  Thanks to Dirk Ullirch for the patch.
Added Text.Pandoc.Compat.TagSoupEntity.

This allows pandoc to compile with tagsoup 0.13.x.
Thanks to Dirk Ullrich for the patch.
Revert "Textile reader: Removed raw LaTeX parsing."

This reverts commit bb61624bb2bba416e1992ecdf101f9660a3edcae.
Apparently someone put this there for a reason, since it's in
the test suite.