Quick

Configuration: Add example filters

Supports #134 by allowing users to specify example filters on
Quick.Configuration. See `FocusedTests.swift` for sample usage.

Filters come in two forms:

1. **Inclusion** filters: Only examples that meet the criteria specified
   by these filters are run.
2. **Exclusion** filters: Examples that meet the criteria specified by
   these filters are not run.

By default, Quick is configured to only include examples where
`["focused": true]`.

But example flags are empty by default, so the condition that
`["focused": true]` is false by default. This means that, by default,
Quick runs no examples, right? Wrong! Because Quick.Configuration is
also initialized with `runAllWhenEverythingFiltered` as true. In other
words, in the case that **no examples** escape the filters, **all
examples** are run.

All this is pretty much exactly how RSpec does things:

- https://www.relishapp.com/rspec/rspec-core/v/3-1/docs/filtering/inclusion-filters
- https://www.relishapp.com/rspec/rspec-core/v/2-0/docs/filtering/run-all-when-everything-filtered