Support IRQ triggering for more efficient MFRC22 tag reading

[Ramblurr]

(this is a feature request)

The python rfid scanner library pi-rc522 has a really cool feature.

Instead of constantly polling for rfid tags to appear, you can call rfid.wait_for_tag(timeout). It uses an IRQ pin to receive an interrupt when an rfid tag has appeared.

This is quite handy, and would be nice to have in diozero.

[mattjlewis]

Thanks, will take a look - lots of improvements to be made to my MFRC522 implementation...