Maintain order of appearance in footnotes

[Crozzers]

This PR closes #533 by making sure footnotes maintain their order of appearance in the final HTML.

Previously, the order of the final footnotes list was based on the insertion order of footnote_ids. This array was populated as the parser processed the text and encountered footnotes. The problem is that markdown isn't processed entirely in order, leading to a disorganised footnotes list being generated.

This PR changes this by instead using the insertion order of footnotes, which is populated at the beginning of the conversion process by _strip_footnote_definitions.

The result is that the generated list of footnotes will appear in the same order as the footnote definitions.

[nicholasserra]

Nice, thank you!

[radiofan]

Hello!
Maintain order of appearance in footnotes is really wrong...
I try execute this code

import markdown2

print(f'Markdown Version: {markdown2.__version__}')
print('-----')

MD = """
aboba [^1]
biba [^2]
boba [^3]
   
 - 1
   
    - 1.1 [^4]
   
 - 2
   
    - 2.1 [^5]
   
 - 3
   
    - 3.1 [^6]
   

[^1]: aboba
[^2]: biba
[^3]: boba
[^4]: 1.1
[^5]: 2.1
[^6]: 3.1
"""

extensions = ["footnotes", "tables", 'cuddled-lists']
extension_configs = {}

print(markdown2.markdown(MD, extras=extensions))

and result is

Markdown Version: 2.5.1
-----
<p>aboba <sup class="footnote-ref" id="fnref-1"><a href="#fn-1">4</a></sup>
biba <sup class="footnote-ref" id="fnref-2"><a href="#fn-2">5</a></sup>
boba <sup class="footnote-ref" id="fnref-3"><a href="#fn-3">6</a></sup></p>

<ul>
<li><p>1</p>

<ul>
<li>1.1 <sup class="footnote-ref" id="fnref-4"><a href="#fn-4">1</a></sup></li>
</ul></li>
<li><p>2</p>

<ul>
<li>2.1 <sup class="footnote-ref" id="fnref-5"><a href="#fn-5">2</a></sup></li>
</ul></li>
<li><p>3</p>

<ul>
<li>3.1 <sup class="footnote-ref" id="fnref-6"><a href="#fn-6">3</a></sup></li>
</ul></li>
</ul>

<div class="footnotes">
<hr />
<ol>
<li id="fn-1">
<p>aboba&#160;<a href="#fnref-1" class="footnoteBackLink" title="Jump back to footnote 1 in the text.">&#8617;</a></p>
</li>

<li id="fn-2">
<p>biba&#160;<a href="#fnref-2" class="footnoteBackLink" title="Jump back to footnote 2 in the text.">&#8617;</a></p>
</li>

<li id="fn-3">
<p>boba&#160;<a href="#fnref-3" class="footnoteBackLink" title="Jump back to footnote 3 in the text.">&#8617;</a></p>
</li>

<li id="fn-4">
<p>1.1&#160;<a href="#fnref-4" class="footnoteBackLink" title="Jump back to footnote 4 in the text.">&#8617;</a></p>
</li>

<li id="fn-5">
<p>2.1&#160;<a href="#fnref-5" class="footnoteBackLink" title="Jump back to footnote 5 in the text.">&#8617;</a></p>
</li>

<li id="fn-6">
<p>3.1&#160;<a href="#fnref-6" class="footnoteBackLink" title="Jump back to footnote 6 in the text.">&#8617;</a></p>
</li>
</ol>
</div>

[Crozzers]

@radiofan FYI I've submitted #612 to fix this