Add firrtl.options

This adds a new package, "firrtl.options", that provides a framework for
working with options inside and outside FIRRTL.

Small changes:
  - Make TerminateOnExit return the correct exit code
  - Deprecate mutable TerminateOnExit
  - Add immutable DoNotTermianteOnExit

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>