Ask user confirmation before building/deploying

[nlewo]

Currently, when comin fetches a new commit, it builds and deploys it unconditionally. This is nice on servers, but I don't find this convenient on my laptops.

• When i'm using my laptop, i sometimes don't want to have to build a new system.
• When i boot my laptop, comin builds and deploys new configuration while as a user, i would prefer to use my computer and choose to deploy later.

I think we could add a configuration option to make comin waiting for a user confirmation before building or deploying new commits.

Ideally, comin should be able to know if a commit has to be built or if it only need to be substituted: i think it would be ok to unconditionally substitute since it is doesn't consume CPU.