Support pluggable loaders

There should be a way for external users to define plugins that produce `Suite`s based on input files. It will need access to the `unittest` API (#48) for things like loading Dart files—it shouldn't need to re-invent the browser bootstrapping logic.

Start by reading the unittest API work in issue #48 and the existing browser bootstrapping behavior. Clarify the external plugin contract for producing Suites from input files, and consider the work done when a loader can use that API without reimplementing browser setup.

Written by the indexing model from the issue text.