Allow control of Lens generator style to match Aether style

[travis-leith]

Currently, myriad produces lens setter with signature 'b -> 'a -> 'b. Aether provides a bunch of utilities for working with Lenses, but does not actually create the Lenses. It assumes setters have the signature 'a -> 'b -> 'b.
I would be nice if I could use myriad to generate Lenses which can be used by Aether.
Maybe an additional attribute to indicate the lens style?

[7sharp9]

Done via #110