add support for image variants

[errordeveloper]

e.g. --variants '{"one":{"foo":"1","bar":1},"two":{"foo":"2","bar":3}"' will build ${image}:${tag}-{one,two}.

[errordeveloper]

This will have to be done as part of config file (#14), as otherwise rebuilding logic will have to cater for changes in variant args and that should be done on more general basis of config file changes.