Markdown summary should not include content footnote

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.
Merge pull request #885 from avaris/markdown-summary-footnote

Markdown summary should not include content footnote