wish: expose 'extensions' from interpret

[markstos]

For a use-case like node-config, we'd like to load all supported config files in a specific order.

It would be helpful if rechoir republished the extensions property of interpret so that rechoir could be used both to introspect the supported extensions as well as load them.

Currently interpret needs to be loaded separately for this, which means it ends up appearing in two locations in the dependency tree.

Thanks for considering the update.