Add support for subscriptions

[testeddoughnut]

This adds support for setting a subscription/check pair as a check
dependency.

I don't do much work in ruby, please be gentle =).

Signed-off-by: M. David Bennett mdavidbennett@syntheticworks.com

[cwjohnston]

@testeddoughnut thanks for the pull request! I think what you are doing here makes sense, but we do have concerns about potential performance impact when the API returns a large number of events.

Given that the implementation here requires operators to opt-in using the subscription: prefix, I think we can merge this as-is and document our performance concern as a caveat.

cc @portertech for 👍 / 👎

[majormoses]

bump

[jaredledvina]

Double bumping on this as my team would really love to be able to depend on a subscription check pair. I would be very interested if we had any rough figures on the perf impact (or when to avoid doing this).

[portertech]

As this feature is opt-in (not hitting /events unless a dependency includes "subscription:") I'm a 👍

Merging with the intention of making a few minor changes (to the Ruby not the functionality).