Add mergeDeep function

[dentrado]

This PR adds a mergeDeep function to timm.

I recently replaced seamless-immutable with timm in a project, which gave a nice perfomance boost :) The only thing missing when making the transition was a way to merge deeply (I used Lodash's merge({}, obj1, obj2) as a temporary solution, but that obviously isn't ideal since it always creates a new object).

From what I can tell when running the benchmarks on this branch vs master, adding this had no measurable impact on performance for the regular merge.

[coveralls]

Coverage remained the same at 100.0% when pulling 70c59c6 on dentrado:deep-merge into fe040c5 on guigrpa:master.

[guigrpa]

This is a useful addition to the library, thanks!

[guigrpa]

Shipped in v1.4.0.