add a quiet option to silence the extra logging

the builder is quiet by default. An option would be to show the grunt output only with the ` --verbose` flag.
But I think it is good that it outputs something so you can see what's going on.

Lot of grunt plugins output stuff, if you really want to limit the output you can use something like https://github.com/bahmutov/quiet-grunt

@adam-lynch what do you think?