Added 'system - shell' (!) command.

Examples:

    Score manager - example scores

        1: Blue Example Score (2013)
        2: Étude Example Score (2013)
        3: Red Example Score (2013)

        scores - new (new)

    > !pwd

    /Users/trevorbaca/Documents/abjad/scoremanager

    > !ls

    __init__.py
    __init__.pyc
    boilerplate
    core
    docs
    editors
    etc
    getters
    iotools
    latex
    managers
    materials
    predicates
    scores
    scr
    test
    wizards
    wranglers

    > !ls | grep rs

    editors
    getters
    managers
    wranglers

    > q