Inject customizable beaming algorithm

[jsbean]

Currently, RhythmSpelling shoots out a default beaming. There is not yet any configurability to this algorithm, though there should be.

[jsbean]

Components could add up something like:

protocol BeamingStrategy {
    ...
}

struct DefaultBeamingStrategy: BeamingStrategy { 
    ...
}

extension RhythmSpelling {
    init(rhythmTree: RhythmTree, strategy: BeamingStrategy) {
        ...
    }
}

func makeJunctions(
    _ leaves: [MetricalDuration], 
    strategy: BeamingStrategy = DefaultBeamingStrategy
) -> [RhythmSpelling.BeamJunction] 
{
    ...
}