Pass example metadata to beforeEach/afterEach

> This is a work-in-progress. Don't merge, but feel free to provide feedback or base a more complete implementation on top of it.

Begins work on issue #105, which essentially involves porting RSpec
example metadata to Quick. See: https://www.relishapp.com/rspec/rspec-core/docs/metadata/current-example

- Create ExampleMetadata class to encapsulate data on current
  executing example
- Add beforeEach DSL that takes a closure with a metadata argument
- Add functional test as a proof-of-concept