Version specifiers like `PANDOC_VERSION` and `PANDOC_API_VERSION` are
turned into `Version` objects. The objects simplify version-appropriate
comparisons while maintaining backward-compatibility.
A function `pandoc.types.Version` is added as part of the newly
introduced module `pandoc.types`, allowing users to create version
objects in scripts.
This makes use of tasty-lua, a package to write tests in Lua
and integrate the results into Tasty output. Test output becomes
more informative: individual tests and test groups become visible
in test output. Failures are reported with helpful error messages.
