Prawns risotto

Notable changes:

• Support for cancelable tasks (p.e. ember-concurrency's tasks) in the search action! 🎉 🎉 If a search is ongoing but the component doesn't care about it anymore (because the select was closed or because a newer search was fired), instead of just ignoring that search, not EPS attempts to cancel it and save some potentially expensive work to be done.
• selectChoose and selectSearch now throw helpful errors when either the select or the option cannot be found.
• The documentation page got some nice syntax highlighting in the code examples.