Unify `deploy` and `select` subcommands

[edudobay]

This also relates to #15 — the solution should consider both points.

• deploy always yields a new build from a git ref, and then deploys that build, if it completes successfully
• select only deploys an existing build

Maybe make it something like this:

laika deploy {--select | --ref GIT_REF | --build BUILD_ID }

We could also add a --default option that would fetch the latest version of a git ref (specified in the settings file) and deploy a new build from it.

laika deploy --default

[default_build]
ref = origin/master