Allow using Pandoc filter in LaTeX commands

[madman-bob]

Following conversation with @ohad, where he explained that you cannot use Verbatim environments inside LaTeX command calls.

I moved the Katla snippet declarations to the headers, so the only thing left in the document body is the command call, and added a test that the Pandoc plugin interacts well with commands. This does mean intermediate LaTeX looks a little strange without an explicit template, but it still works fine.

As a nice side-effect, this also gets rid of the extra spaces I complained about when I first made this plugin.

[ohad]

Nice, thanks!
I would prefer not to add pandoc to existing ubuntu-ci dependencies, but instead create another copy of CI, say pandoc-ubuntu-ci which will have this dependency.

(Pandoc tests should be skipped if pandoc is not installed.)