Added functionality for the tally verb

[willdebras]

The commits add support for the tally verb by doing the following:

• Creating a function to prep specs for the tally verb, mimicking the count verb, by allowing calculation of totals with or without group mappings
• Adding support to the the datamation_sanddance() function to allow the passing of of tally() in a sanddance string, e.g.: "small_salary %>% group_by(Degree) %>% tally()" %>% datamation_sanddance()

[ghost]

All CLA requirements met.

[willdebras]

@jhofman Wanted to get a quick verify I should go ahead and merge this. You mentioned AOK to merge in our last meeting, but wanted to verify I should be making merges and check in about review/QC process.

[jhofman]

@willdebras, yes all good in terms of review process.

one late-occurring thought on this: is it in any way safer or easier to just map the "tally" function to prep_specs_count, since it's just an alias? could cut down on some redundant code? (hadn't thought of that until now.)

[willdebras]

I've thought a bit about if we want to integrate tally in a way that just aliases prep_specs_count, but I think it makes sense to leave them as two separate files. Since prep_specs_count is returning to-be-json response, I don't want to retroactively update that response with the custom animation metadata or the title metadata and I think it would muddy code a bit to add additional parameters in the do.call on the call_verb in datamation_sanddance() to get the specification for these. I think pretty clean to just have a separate file for each. I'm going to go ahead and merge.