Add threshold feature that enables non-zero exit code if metric is below set value

[aaronfowles]

It would be useful to specify a threshold for metrics so that CI could detect if a metric has fallen below a certain threshold. Eg:

wily build src/
wily rank --threshold=80

if the Total value is less than the threshold, a non-zero exit code will get chucked out that the CI can pick up.

[aaronfowles]

Ah, it seems this is probably a sub-set of #66 .

[Anthony Shaw]

Hi @aaronfowles I agree this would be a useful feature. there's a WIP PR in the backlog, I'll chase the author and we'll see if we can get it merged