Run idempotently when resuming a previous run

[mcmire]

If the tool aborts midway through a run and somehow the release spec is not removed, the user should be able to re-run the tool to complete the full run. In this case, anything that's happened in the previous run should not cause the tool to abort again.

• if a package has already been bumped, ignore and continue
• if the release branch has already been created, ignore and continue
• if we've already switched to the current branch, ignore and continue

[mcmire]

This ends up conflicting with #17, as the tool should not be in a mixed state if it encountered an unexpected failure while executing the release spec. If the release spec file still exists somehow after the tool stops running, then the tool should throw an error. This should be addressed via #24. In light of these two other issues, I'm closing this.