This new module exports `pandocVersion` and `pandocVersionText`,
which are no longer exported from Text.Pandoc.Shared. [API change]
Also, we now set the `pandoc-version` variable centrally rather
than in the writers. One effect is the man writer now emits
a comment with the pandoc version (this was intended before,
judging from the template, but it didn't happen because the
vairable wasn't set).