Markdown instance carries state for subsequent uses. Content
and summary parsing is done with the same instance. Since
footnotes are processed with an extension and stored as state,
content footnote is duplicated for summary.
This PR adds a ``.reset()`` call before summary parsing to clear
the state. It also adds a test case with footnotes.
More precisely, group tags or categories without considering the case.
This fixes the bug where two categories with just the case as difference were
considered as distinct, but generate the same file: one overwriting the other.
Thanks to @Avaris for helping with the tests.
Tempdirs should have useful prefixes to aid in collecting information
about failed tests.
Implicit concatenation is better than backslash line continuation.
