Allow user to register executor for given executable name

From https://github.com/gradle/exemplar/pull/22#discussion_r208434792:

Allow exemplar/user to register some extension if one wants to customize how they run "gradle" or "groovy" or "bash". For example, they could run "ruby" with specific RVM config.

No files, tests, or entry points are named in the issue. Start by locating how Exemplar runs executables and where user extensions can be registered; done means a user can register behavior for a named executable, including running ruby with a specific RVM configuration.

Written by the indexing model from the issue text.