Keybase support

Setting up the GPG toolchain is cumbersome (on OSx even more so). With [keybase.io](https://keybase.io) things got a lot easier recently. That's why I was wondering if there's an interest in bringing `keybase` into `boot`.

Changes to the existing `boot.pod.gpg` namespace would be marginal. I've already hacked something together that hooks into the existing `boot.core.built-in/push` task. Actually it `alter-var-root`s `boot.pod.gpg/sign-jar` to `instilled.boot.keybase/sign-jar` in a task, see [here](https://github.com/instilled/boot-ext/blob/master/src/main/clojure/instilled/boot/task.clj#L11).

Instead of that hackery, if there's interest into bringing this into boot, we should find a better way to accomplish this. An env var à la `BOOT_SIGN_TOOL={gpg,keybase}` or an additional parameter to `boot.core.built-int/push` and a minor rework of `boot.pod.gpg` would probably just do fine.

Let me know what you think!

Cheers, Fabio

Start with the existing boot.pod.gpg namespace and the boot.core.built-in/push task, then compare the linked instilled.boot.keybase/sign-jar integration. Evaluate whether the proposed environment variable or an additional push parameter provides the needed Keybase selection without alter-var-root; done means Keybase signing works through the normal push flow.

Written by the indexing model from the issue text.