Bug in footnote handling.

If there is a single space on the line following a footnote, the footnote regex gets a bit
confused.

import markdown2

text = '''
Here is some markdown with a footnote[^myfootnote] embedded within it.

[^myfootnote]: I define the footnote here on 
    multiple lines but the line below contains *one* space but looks blank
 

'''
markdowner = markdown2.Markdown()
markdowner.convert(text)

This produces the following html output.

<p>Here is some markdown with a footnote[^myfootnote] embedded 
within it.</p>\n\n<pre><code>multiple lines but the line below 
contains *one* space but looks blank\n</code></pre>\n'

Start with the Python reproduction in the issue and trace markdown2's footnote parsing entry point using the supplied input. Check how a line containing one space is handled after a multi-line footnote. Done means the footnote remains parsed as a footnote and the generated HTML no longer places its continuation in a code block.