User control on predicted label file names

The predict command generates a predicted.label file along with the input tracking data file(s).

Such a file needs to be renamed in the case multiple taggers are applied to the same tracking data files. This can become tricky when these taggers are applied simultaneously.

We need a -o/--output argument for the predict command so that the predicted.label files are renamed accordingly before they are pulled from the backend's "processed" directory.

This feature was requested by Ben Jones.