Allow fancy wrapper tags to set custom fallbacks (or do whatever else!)

[dhartunian]

Seems like this is already supported...just gotta write it but maybe could be a larceny-extras thing or something.

For example, we could replace:

<bind tag="fallback">I'm missing ^_^</bind>
<missingTag />

with:

<withCustomFallback>
  <fallback>I'm missing ^_^</fallback>
  <content>
    <missingTag />
  </content>
</withCustomFallback>

which can help with scoping the fallback and make it clear within the XML structure!