YUI adaptor for istanbul factored out into its own module.
The index.js file can be used as the --post-require-hook argument to istanbul
in order to hook the YUI loader for coverage.
Note that this module neither depends on istanbul nor on YUI by design.
It only provides the hook function.
Test runners for YUI tests (e.g. ytestrunner) can depend on this module in order
to provide coverage for YUI tests.