Clean up superscript/subscript markup

[Mr0grog]

In #34, I added superscript and subscript support. However, the output is often messier than need be because Google Docs nests the superscript/subscript formatting inside other inline formatting (like bold or italics), so formatting inside the superscript/subscript can break things up into several sup or sub tags.

For example, Google Docs might produce input like the following if only part of a subscript was bold:

<span style="vertical-align: sub;">
  Plain text
</span>
<span style="font-weight: bold;">
  <span style="vertical-align: sub;">
    bold text
  </span>
</span>

Which produces markdown output like:

<sub>Plain text </sub>**<sub>bold text</sub>**

It would probably be nicer to go the other way around and treat the subscript/superscript as the outer layer of nesting if it’s the only thing inside the italic/bold. So we’d wrangle the GDocs HTML from above into something like:

<span style="vertical-align: sub;">
  Plain text
</span>
<span style="vertical-align: sub;">
  <span style="font-weight: bold;">
    bold text
  </span>
</span>

And then merge consecutive <sub> or <sup> elements to ultimately get output like:

<sub>Plain text **bold text**</sub>