Mark added transactions as "imported"

[mKeRix]

I'm a big fan of YNAB's model for entering transactions manually at first and then matching them against bank transactions as they roll in, like they have it for their official bank import feature. According to their docs you can also achieve this via the API. For that the import_id should be set for each transaction, ideally following a format convention they describe.

I think that this would add nice value to this project. I'm also willing to try implementing this once I have received my FinTS registration.

[bahlo]

Hey Heiko, thank you for creating the issue! Sounds like a great thing to add. Feel free to take a look (and contact me if you run into problems) 🙂