Improved versioning.

OLD:

    > vdls

    0001.ly 0001.pdf 0001.py
    0002.ly 0002.pdf 0002.py
    0003.ly 0003.pdf 0003.py

NEW:

    > vdls

    illustration_0001.ly illustration_0001.pdf output_0001.py
    illustration_0002.ly illustration_0002.pdf output_0002.py
    illustration_0003.ly illustration_0003.pdf output_0003.py