Perform template substitution when parsing default values

[mkmckenz]

For reference, here's how I've patched for issue #342. This change worked for me, but I won't claim it's the correct fix.

[saudet]

Looks good! Thanks. The Parser is pretty ad hoc at the moment, so whatever works is probably "correct". :) Long term, the idea is to use something like Clang, see issue #51.