Support the :-> shorthand

[valerauko]

A very common pattern is to just extract one value from the database, which in re-frame I'd do with :->

(reg-sub
 ::my-value
 :-> :some-keyword)

Writing just :some-keyword results in the subscription returning the parameter vector just like in re-frame, so supporting this syntax would be very helpful.

cf https://github.com/day8/re-frame/blob/dd18854498b6a5e51ed09bfcbd193d9dd84d8381/src/re_frame/core.cljc#L259-L271

I'd be happy to help implementing this

[wkok]

Interesting, I've never used this syntactic sugar before, but do see its benefit.
You're welcome to submit a PR for it