Optional core.async helpers

[metasoarous]

It would be nice to have something like slurp-async (see #54) that puts each row one by one on a go channel, perhaps using the transducers and pipeline. I don't want to make a hard requirement on core.async though, so we should follow the route of mpdairy/posh's Reagent plugin, so that the relevant function(s)/ns(s) are only defined if core.async is available. I don't know if this means a separate ns or not; Probably best not to define one unless necessary (or unless we realize there are enough such helpers that it actually makes sense to, which I doubt will be the case, since a lot of the logic gets generalized nicely with transducers).

(extracted from #45)