Complete [runtime] type coercion. Closes #443.

Addresses #443.

Used in lieu of [runtime] validation in config.py, and in the broadcast
command to coerce command line string values to the right type. This
also corrects a bug in the broadcast command: the try block must be
inside the coercion loops or it aborts at the first KeyError.

@dpmatthews & co - plz review.