Add support for caching between builds

[MOZGIII]

Right now cache key is $DRONE_JOB_NUMBER.
Can we add the ability to use branch as a cache key instead?

[mjwwit]

Maybe we should make it completely configurable (with sensible defaults). I imagine a situation like:

pipeline:
  restore-cache:
    image: drillster/drone-volume-cache
    restore: true
    mount:
      - ./node_modules
    volumes:
      - /tmp/cache:/cache
+   cache_key: [ BRANCH, JOB_NUMBER ]

Which would result in:
/cache/$DRONE_REPO_OWNER/$DRONE_REPO_NAME/$DRONE_BRANCH/$DRONE_JOB_NUMBER

Are you comfortable with some bash scripting? If so you could try and make a PR. If not, I'll work on it when I get around to it.

[MOZGIII]

Yes, I like the idea.
I can do bash, but I won't be able to get into it soon (probably I'll do it in our next CI optimization pass), so I'd propose you do it if you want to.

[mjwwit]

Alrighty, I'll add it to our backlog then.