Allow feedstocks to opt out of PRs (version and/or migration)

[CJ-Wright]

We may need a way for feedstocks to opt out via a blacklist. We'll still keep track of the feedstocks in the graph, but we can remove them from the PR process.

[jakirkham]

How was this solved with bob.*?

[CJ-Wright]

https://github.com/regro/cf-scripts/pull/116/files#diff-115a84c4eab882f7b57c4360003383d9R287

[jakirkham]

Ah ok. So not feasible for this problem. Hmm...

[CJ-Wright]

Adding a blacklist is easy, the main thing is putting the blacklist where users can get at it. That way they can put PRs in to blacklist their packages rather than us having to put them in.

[CJ-Wright]

We might consider making the blacklist a yaml file, where keys are the feedstocks to ignore and then values are some statement about why we are ignoring them. This will allow us to take things off the blacklist in the future when their issue is resolved.
eg

git: lack of multi source support