Improved (pdfcp) user experience.

The command now tells you what it is going to do before doing so.

Example:

    Red Example Score (2013) - build files

       1: back-cover.pdf
       2: back-cover.tex
       3: draft.pdf
       4: draft.tex
       5: front-cover.pdf
       6: front-cover.tex
       7: music.ly
       8: music.pdf
       9: preface.pdf
      10: preface.tex
      11: score.pdf
      12: score.tex
      13: segment-01.ly
      14: segment-02.ly
      15: segment-03.ly

          files - copy (cp)
          files - new (new)
          files - remove (rm)
          files - rename (ren)

    > pdfcp

    Will copy ...

     FROM: /Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/segments/segment_01/output.pdf
       TO: /Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/build/segment-01.pdf

     FROM: /Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/segments/segment_02/output.pdf
       TO: /Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/build/segment-02.pdf

     FROM: /Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/segments/segment_03/output.pdf
       TO: /Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/build/segment-03.pdf

    Ok? y

    Red Example Score (2013) - build files

       1: back-cover.pdf
       2: back-cover.tex
       3: draft.pdf
       4: draft.tex
       5: front-cover.pdf
       6: front-cover.tex
       7: music.ly
       8: music.pdf
       9: preface.pdf
      10: preface.tex
      11: score.pdf
      12: score.tex
      13: segment-01.ly
      14: segment-01.pdf
      15: segment-02.ly
      16: segment-02.pdf
      17: segment-03.ly
      18: segment-03.pdf

          files - copy (cp)
          files - new (new)
          files - remove (rm)
          files - rename (ren)

    >