Add option to allowlist decorated functions as un-dead

[Tadaboody]

Certain decorators like Flask.route point to usage from some external code or framework.
I'd like the ability to create a per-project whitelist (maybe with some common defaults) of such decorators to reduce the number of false positives

[asottile]

seems reasonable, might be difficult / complicated to get this right given someone can myapp = Flask(...) ... @myapp.route or even from foo import myapp ... @myapp.route