Implemented __getitem__() on lilypondfiletools.Block.

Example:

    >>> block = lilypondfiletools.Block(name='score')
    >>> score = Score(name='Red Example Score')
    >>> block.items.append(score)
    >>> block['Red Example Score']
    Score(is_simultaneous=True)

This makes it easy to retrieve block items by name.

Note that ...

    * lilypondfiletools.LilyPondFile.__getitem__()
    * lilypondfiletools.Block.__getitem__()

... work in parallel.

Use the first method to get a block and then use the second method to get an
item in the block.

Pattern:

    >>> score_block = lilypond_file['score']
    >>> score = score_block['Red Example Score']
    Score(is_simultaneous=True)