Improved remove*() user experience.

The user interaction now includes an explicit listing of paths to be removed
prior to user confirmation.

Example:

    Red Example Score (2013) - build directory

        1: red-example-score-segment-01.ly
        2: red-example-score-segment-01.pdf
        3: red-example-score-segment-02.ly
        4: red-example-score-segment-02.pdf
        5: red-example-score-segment-03.ly
        6: red-example-score-segment-03.pdf
        7: red-example-score.pdf

        back cover latex - edit (bce)
        back cover latex - generate (bcg)
        back cover latex - typeset (bct)
        back cover pdf - open (bco)

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

        front cover latex - edit (fce)
        front cover latex - generate (fcg)
        front cover latex - typeset (fct)
        front cover pdf - open (fco)

        preface latex - edit (pfe)
        preface latex - generate (pfg)
        preface latex - typeset (pft)
        preface pdf - open (pfo)

        score latex - edit (se)
        score latex - generate (sg)
        score latex - typeset (st)
        score pdf - open (so)

        segment assembly ly - edit (sege)
        segment assembly ly - lilypond (segly)
        segment assembly pdf - open (sego)

        segment lys - copy (lycp)
        segment pdfs - copy (pdfcp)

    > rm

    Enter files(s) to remove> 2, 4, 6

    Will remove ...

        /Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/build/red-example-score-segment-01.pdf
        /Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/build/red-example-score-segment-02.pdf
        /Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/build/red-example-score-segment-03.pdf

    Type 'remove 3' to proceed> remove 3