Make ExsclaimTool and JournalFamily configurable objects

[trevorspreadbury]

Is your feature request related to a problem? Please describe.
This is not a problem now, but should it arise, I'd like a potential solution documented. If many distinct contributors wish to use exsclaim using specific ExsclaimTool subclasses that don't make sense to merge into the repo, they would be forced to create forks which is not ideal in many cases.

Describe the solution you'd like
Allow configurable subclasses. You can use traitlets Configurable objects to allow customization via config files.

Describe alternatives you've considered
Doing nothing -- the best bet before this arises