This should make it easier for people to write custom composefile
parser without duplicating too much code. It takes the default
transformers and any additional number of transformer for any
types. That way it's possible to transform a `cli/compose` map into a
custom type that would use some of `cli/compose` types and its own.
Signed-off-by: Vincent Demeester <vincent@sbr.pm>
0246bc1b3b