Add JSON output type to grandma diff

[gsf4726]

Currently grandma.diff output only supports tabular text like:

Latencies:  mean               50                 95                 99                 max

run: someTest, 10s, 1 concurrent, 3 total
fullTest    4650.651ms (-35%)  4686.128ms (-33%)  4880.873ms (-33%)  4880.873ms (-33%)  4880.873ms (-33%)

bench: someTest, 10s, 1 concurrent, 3 total
fullTest    3456.642ms         3527.152ms         3674.297ms         3674.297ms         3674.297ms

Adding a JSON output option would make it easier for an automated process to handle the results.

[catdad]

Hmm... what info are you interested in having in a JSON diff? You can already get a JSON for individual runs. What would a diff JSON look like?

[gsf4726]

Was primarily thinking, to implement a failure threshold in automation, it'd be convenient to get the deltas that are currently reported in the diff text output.

Certainly a convenience though.