Stricter quoting within "command:" rules

Objects (and target_name) must not be quoted, files (and file-like
targets) must be quoted.  This is entirely unnecessary for running
maker, but it establishes a connection between the code in the
makerfile and the function that would be run.
Remove the 'rule:' interface in favour of 'command:'

This is a breaking change, but I don't think anyone is using this yet.
The rule interface offers no real benefits over the command one, and
the command one is way way simpler/shorter and more expressive.

I might rename these keys very soon, which is yet another breaking change.