If --message is not specified, prompt for it when it is needed

[giltayar]

So that --dry-run (and others) can be executed without --message.

The idea is this: in command-build.js, if message is undefined, then wait for the execution of the during phase, and just before it, ask for the message (using the inquirer package from NPM) and update the options.

We're doing it before the "during" phase and not the after phase, because some jobs need the option during the during phase (for example, Roundforest's deploy job.