Script the signing procedures as much as possible

[conorsch]

We have a few sources of documentation describing the workflow to generate new rulesets and the corresponding signature. Many of the steps are manual commands that can easily be scripted. Since we use an airgapped signing process (as described in https://github.com/EFForg/https-everywhere/blob/master/docs/en_US/ruleset-update-channels.md#signing), we can't script everything, but we sure can put the process on rails.

While working on #19 I cobbled together a crude script that I'll submit as a draft for review.

[emkll]

Let's consider this closed via #23 . We can open more specific issues for signing/release automation in the future, as necessary.