Added 'definition.py - illustrate' (di) segment command.

The command implements the artifact candidacy protocol.

The command will replace __make__.py usage.

Note that all __make__.py commands are deprecated.

Example:

    Red Example Score (2013) - segments - A

       1: __init__.py
       2: __make__.py
       3: __metadata__.py
       4: definition.py
       5: illustration.ly
       6: illustration.pdf
       7: versions/

          __make__.py - edit (ke)
          __make__.py - interpret (ki)
          __make__.py - stub (ks)

          definition.py - edit (de)
          definition.py - illustrate (di)
          definition.py - stub (ds)

          illustration.ly - edit (ie)
          illustration.ly - interpret (ii)
          illustration.pdf - open (io)

    > di
    The PDFs ...
        /Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/segments/segment_01/illustration.candidate.pdf
        /Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/segments/segment_01/illustration.pdf
    ... compare the same.
    Preserved /Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/segments/segment_01/illustration.pdf.

    >