Support backlinks in notmuch email messages

[Zetagon]

I've discussed this before on the discourse https://org-roam.discourse.group/t/how-easy-would-it-be-to-extend-org-roam-with-backlinks-to-other-types-of-items/1582 . I'm not sure if this is a feature that should be in org-roam so I'm leaving this as a draft for now (the commit messages aren't the best either).

[jethrokuan]

Support for external caching (non-Org) should be placed into the contrib section. Concretely that means org-roam-db.el needs to expose a way to consider and parse other types of files (beyond what org-roam-list-files). I'll think of a way to expose such an interface, but the current modifications are not ideal.

[cooladam201212]

I think this is a very useful feature, there's even a stand-alone MacOS app called Hookmark to achieve this.