Add and include peer dependencies

[HyperBrain]

What did you implement:

Relates to #230
Closes #223

How did you implement it:

When adding external dependencies, their peer dependencies are now included too. This will add dependencies that are missed by Webpack if only the dependency requires its peer, but not the project's code.

Each dependency is checked for peers.

How can we verify it:

Use a project that depends on request-promise but does not use request internally. The project
must depend on request too, because it is a peer dependency of request-promise.

Todos:

• Write tests
• Write documentation
• Fix linting errors
• Make sure code coverage hasn't dropped
• Provide verification config / commands / resources
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES
Is it a breaking change?: NO