Add Plan Command

[theory]

I think that, for now, we should ignore the VCS integration and focus on the plan file. This is because I think it will be the mots important way to plan for most folks, as repositories are not usually that fastidious. I myself often make many, many changes to deployment scripts as I am doing development, and really I just want them to start from a certain point, once I've tagged an app to ship.

With this in mind, I think that add-step should be replaced with a plan command. It will take subcommands to do things with the plan file, such as:

• Add a tag
• Add a step
• Add a step and tag
• Generate a plan file from the dependency graph
• Generate a plan from the VCS history

The second-to-last item there assumes there is no plan file, or a new one needs to be created. The last item allows one to bootstrap a plan from VCS history. This is one of the ways in which a VCS can complement Sqitch.

Can probably get rid of the concept of untracked steps this way, too: if it's not in the plan, it is not known.

[theory]

No longer valid. Changed add-step to just add and added the tag command. Other plan-modification commands will likely follow, and not be integrated into an über plan command. I will open separate issues for those.