Add merge-env! function
Add merge-env! function

- Rename previously existing multimethod to pre-env!.
- Rename on-env! multimethod to post-env!.
- Add new multimethod, merge-env-key!, which dictates the merge
  strategy for each key in the env.

crisptrutski/boot,upgradingdave/boot,instilled/boot,boot-clj/boot,kennyjwilli/boot,tobias/boot,junjiemars/boot,kausdev/boot,bbatsov/boot,upgradingdave/boot,crisptrutski/boot,junjiemars/boot,tobias/boot,crisptrutski/boot,kennyjwilli/boot,kausdev/boot,ragnard/boot,RadicalZephyr/boot,ragnard/boot,kausdev/boot,RadicalZephyr/boot,tobias/boot,danielsz/boot,danielsz/boot,upgradingdave/boot,ragnard/boot,instilled/boot,junjiemars/boot,danielsz/boot