Added Selector.with_callback() method. MODEL.

MODEL: Selector.with_callback() lets composers create selectors with arbitrary
callbacks. Many callbacks which might be useful during the component selection
process are also likely very composition-specific. You can now simply create
selector callbacks in your makers subpackage and configure selectors with them
directly.

For example, a composer might wish to partition a sequence of leaves into
subsequences where each subsequence's last leaf has an after-grace container
attached. This functionality is probably too eccentric for inclusion in Abjad,
but should be facilitated nonetheless. Just create a SelectorCallback which
encapsulates this partitioning logic and aggregate it into a Selector via
Selector.with_callback().