support a nice way of changing the learning rate

[SobhanMP]

It would be nice to have a easy and rule independent way of changing the learning rate like in Flux.jl.

Right now the best way i can think of is making another state tuple while changing the first argument but then changing the learning rate is not rule independent.

@mcabbott suggested

Although perhaps there ought to be one, like st = setup(rule, st) without zero-ing the momenta. Maybe make an issue?

related: #15 (seems to be not active)

[mcabbott]

See what you think of #89. I made it a new verb, although perhaps it could in fact be made a method of setup --- the signature of the first call doesn't really distinguish the two uses, but the state tree contains Leaf or nothing, and never contains new parameters (i.e. things with isnumeric(x)).

[SobhanMP]

Thanks, that was fast and looks pretty good. I personally don't care but making :eta a parameters might be good idea.