"enhance": don't rewrite .dix?

I tried [enhance](http://wiki.apertium.org/wiki/Dixtools:_Enhance), which seems handy, but it completely rewrites the .dix file, which leads to unnecessarily large diffs. It also strips the `c` attribute.

It'd be nice if enhance could either

1. just output the new additions to the output file, or
2. only add the new words to the .dix file with no other changes (e.g. at the end of the section or after the words they're based on)

Start by locating the enhance entry point and how it reads and writes .dix files. Investigate whether it can emit only additions or preserve the existing file while adding new words, including the c attribute. Done means one of those requested behaviors is implemented without unrelated rewrites or attribute loss.

Written by the indexing model from the issue text.