Add a new way of formatting our output in a consistent way. This turns
print_list and print_dict into a formatter that has the same API as any
other formatter and allows users to create their own formatters and plug
them into cratonclient.
This includes tests for the base level formatter and our two default
formatters as well as some refactoring to allow users to specify their
own --format.
At the moment, however, the subcommand shells do *not* use the pluggable
formatter decided by the user. That change and all of the downstream
effects it has on testing is going to be *very* significant and deserves
its own commit as this one is large enough.
Change-Id: I6649ebce57d5ddf2d4aeb689e77e3c17ef3a2e97
d1a88bad18