Rework SubjectTransform

Filename is now accessible via the metadata interface, and Subject is now no
longer an important variable name.
Rework subject transform to apply an input regex to the value of an input key
name, and store the resulting value in the specified output key name.
Apparently I'm going to be held responsible for this
Merge pull request #91 from biometrics/subject_transform

Rework SubjectTransform