Check for required extensions

[itowlson]

The CNAB WD allows a bundle to say "any tool that wants to run me has to support the following non-core extensions." Tools are meant to check this and decline to run the bundle if there are any required extensions that they don't support. It would be good if self-installers could do this.

In practice this is not as important as it sounds because the self-installer works by invoking Duffle, and Duffle will safely fail the install for us.