Support pluggable launchers

It should be possible for developers to create custom launchers for browsers that users can then include, like other plugins. These launchers would presumably have a similar API to the existing `Browser` interface: a way to create an instance that's pointing to a given URL, monitor that instance's uptime, and close it if necessary.

This is blocked on #48.

Start by reviewing the existing `Browser` interface and issue #48, which blocks this work. Determine the API needed for custom browser launchers to create an instance for a URL, monitor its uptime, and close it when necessary. Done means developers can provide launchers as plugins and users can include them like other plugins.

Written by the indexing model from the issue text.