Additional coverage can be checked after the runn run

[k2tzumi]

refs #748

Support --coverage-out otpion

$ runn run --shuffle on --shard-n 2 --shard-index 1 --coverage-out coverage.json path/to/**/*.yml

How about having the output in json format?
Most of the processing accounts for runbook load time, and having the coverage output in post-processing at test time uses less computer resources than running it separately!

[k2tzumi]

I tested it on a project that already had runn installed, but it took about 4.5 minutes just for static analysis. 😅

[k1LoW]

I think it's a good developmental feature!