Add MDES 3.2 support to mdes-console.
Make pp available by default in the console.
Implement Variable#diff. Starting #10.
Implement TransmissionTable#diff. #10.

Includes differences in the corresponding variables.
Implement VariableType#diff. #10.

Also extends Variable#diff to include Variable#type.
Move CodeList and CodeListEntry out of VariableType. #10.

Since CodeListEntry is going to be carrying more logic once #diff is
implemented on it, it makes sense to put it in a separate file.
Implement CodeListEntry#diff. #10.

Also extends VariableType#diff to include code list differences.
Implement Specification#diff. #10
Introduce concept of strict and not diffs. #10.

The default (non-strict) diff attempts to only include meaningful
changes. It ignores:

* Changes to whitespace, punctuation, or case in code list labels
* Variable status changes which are not actionable
* The unused "global_value" and "master_cl" attributes on CodeListEntry

The strict diff includes all these things along with the meaningful
changes.