Allow passing an array of resolver objects to makeExecutableSchema

[mfix22]

• Closes Why should i merge resolvers manually when typedefs are automatically merge ?  #486
  @stubailo here is the PR. I just wanted to get initial feedback before:

1. Copying over lodash.merge and adding local type defs (this is my first time ever writing Typescript 😄)

• Note: there are already 16 transient dependencies on other lodash modules
• The issue here in not the copying (hehe), but rather writing the typedefs for what turns out is a 2000 line file (mostly comments albeit).

2. Implementing similar functionality in mergeSchemas
3. Updating CHANGELOG.md

TODO:

• If this PR is a new feature, reference an issue where a consensus about the design was reached (not necessary for small changes)
• Closes Why should i merge resolvers manually when typedefs are automatically merge ?  #486
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass
• Update CHANGELOG.md with your change

[stubailo]

This is a great idea but let's avoid using lodash.merge.

We should be able to get a similar result in a much simpler way by doing two loops over the input objects.

[stubailo]

I don't think we need lodash.merge - we could just write a simple function that loops through a few objects and merges them. Shouldn't be more than 10-20 lines of code I think since we don't need to support all the stuff that lodash.merge does.

We've intentionally kept the lodash dependency out.

[freiksenet]

There is https://github.com/apollographql/graphql-tools/blob/master/src/stitching/mergeSchemas.ts#L306-L322. You can extract it and use it for merging.

[mfix22]

cool thanks for the review guys! Let me know if you want me to move mergeDeep to it's own utility file.

[freiksenet]

I've combined your two PRs and will merge them together.