Add slurp-async so cljs has a slurp analog

[metasoarous]

Will need to load a csv parser. Papa parse seems popular in js land, and it's in cljsjs repos. I tried to get it to work on another project once, but couldn't quite figure out the whole cljsjs thing (could have tried harder I'm sure).

This should probably be defined for both clj and cljs, since why not.

(extracted from #45)