Add support for multitenant deployments

[deesel]

In order to provide multitenancy we must differentiate alerts from different customers. Original blackout plugin takes customer field into account when evaluating blackouts, alerta-blackout-regex now does too.

[mirceaulinic]

Looks good, thanks for the PR.

Would you mind adding a quick test as well? :)

[deesel]

Looks good, thanks for the PR.

Would you mind adding a quick test as well? :)

Absolutely, please see if it works out for you.

[mirceaulinic]

Thanks @deesel! That works very well. There are some CI failures, but it's unrelated to your changes, I shall fix them soon.
I hope to release a new version in the follwoing days.