WIP Support for :skipif:

[twmr]

• currently only supported in the testoutput directive
• completely skips the doctest if the expression in :skipif: evaluates to True.

It is planned to extend this commit s.t. the behavior of skipif is the same as written in the documentation of sphinx.ext.doctest:

The value of the skipif option is evaluated as a Python expression. If the result is a true value, the directive is omitted from the test run just as if it wasn’t present in the file at all.

Closes: #9