Localize on demand

This changes the `carto` command line tool to avoid calling millstone by default for mml files.

Usually people hooking into `carto` from the command line do not need the millstone calls because they are very TileMill specific. If TileMill were needed anymore `carto` from the command line likely would not be being used!

This is a followup to the original approach by @danzel in #143. It solves mapbox/millstone#88 (or at least avoids the understandable WTF).

To work nicely after this is landed we should also likely move on mapbox/tilemill#1884.

But for specific projects the one-time fix is easy: gravitystorm/openstreetmap-carto#32