Add generic type overwrite to deepmerge.all

[DanielMSchmidt]

Hey,

Thank you for this great library, I just happened to have a case where deepmerge.all came in handy and I wanted to have the types set up right, so this is why I would need this change.

[TehShrike]

This makes sense to me.

In this pull request could you also add a line to https://github.com/KyleAMathews/deepmerge/blob/master/test/typescript.ts that fails the type checker without this addition?

[DanielMSchmidt]

Oh, I have completely overlooked this one. Added it 👍

[TehShrike]

Whoops, looks like it's failing at the moment:

test/typescript.ts(55,1): error TS2322: Type 'object' is not assignable to type '{ wat: number; }'.
  Property 'wat' is missing in type '{}'.
test/typescript.ts(56,1): error TS2304: Cannot find name 'merged4'.

https://travis-ci.org/KyleAMathews/deepmerge/jobs/465025267

[DanielMSchmidt]

Sorry for the extra feedback loop 🙈

[TehShrike]

Published as 3.1.0