Support pluggable reporters

There should be a means for third parties to create reporters that are dynamically loaded and used by unittest.

This is particularly tricky because reporters are likely to be desired on a user-by-user rather than project-by-project basis, so there's some friction to just putting it into a project's pubspec.

The issue names no files or tests. Start by tracing how unittest currently selects and loads reporters, then clarify the user-level configuration and loading behavior needed for a third-party reporter to work without a project pubspec change.

Written by the indexing model from the issue text.