feat: add commands for publishing rc

[achingbrain]

This change adds two commands:

aegir update-last-successful-build

This command updates a branch called build/last-successful (configurable)

It's intended to be run on CI after every successful build of master.

It:

1. Checks that it's running on the head (or detached head) of master
2. Creates a temporary branch
3. Creates yarn.lock and npm-shrinkwrap.json
4. Adds them to git
5. Deletes the remote build/last-successful branch if it exists
6. Pushes the new build/last-successful to origin

It deletes the remote branch first because merging conflicts is tedious and
this branch should only be used by automated processes.

aegir publish-rc

This creates a release branch for the next release and publishes a release candidate to npm.

It:

1. Deletes your local copy of build/last-successful
2. Pulls down and checks out an updated version
3. Creates the release branch by semver.incing the package.json version
4. Overwrites existing pre-id in the version number
5. Uses aegir's release command to publish an .rc version

[achingbrain]

Just used to publish https://www.npmjs.com/package/ipfs/v/0.38.0-rc.0

[achingbrain]

N.b.

1. I'll open more PRs for publishing automation as we go through the process this month
2. You have to run aegir publish-rc (and probably subsequent commands) manually for now as our accounts require OTPs which are hard to automate. Maybe we need a special CI user that doesn't have it enabled, let's talk about it.
3. I'll update the js-IPFS release issue template when these commands are merged