Add framework for dealing with barred chords

[tomcontileslie]

Currently, ranking of diagrams is done by looking at which notes are low, and how "stretched" the chord is (via various metrics). How can we get the algorithm to recognise when a given list of frets can be barred conveniently, giving it extra ranking points? The first observation to make is that this would probably depend on handedness.

The second part of this issue is to then implement a method of drawing bars in all types of outputs (plaintext, PNG, and any others).