Writing prefix tags

[acesnik]

Currently, the writer doesn't write prefix tags. Should we extend the writer to do this?

[rfellers]

Commented on the PR before I saw this ... yes, I'm in favor, but wasn't sure what that should look like. Maybe something like writer.WriteString(bool usePrefixTagIfValid)?

[acesnik]

I think we will have to add a string PrefixTag property to ProFormaTerm to know when to write this back out.