feature: add an abnormal hook for org-roam-extract-subtree #182

[jmay]

#2365

Refactoring: extracted pieces of org-roam-extract-subtree to utility functions.
Added a defcustom for the hook (abnormal hook, takes a parameter). Default is an empty hook, so no change in standard behavior.

For consideration: include 2 sample hook functions, unused by default: one to insert a link to the extracted node, one to insert a #+transclude: line.

[jmay]

Hey @jethrokuan can you see why this test case fails under emacs 27? I haven't use buttercup before, I'm probably relying on some behavior that doesn't exist in older emacsen.

Also for the new test file test-org-roam-extract.el I just replicated the file header from the other tests. Not sure if that's the correct practice, I don't want to invoke your name in there without an OK from you.