Arbitrary custom date format tokens

I had an issue where I needed tokens that were built from custom date formats I'd saved using admin/settings/date-time/formats/custom. I couldn't build up the format using the component tokens as I needed it for fields (not the node as a whole). This patch will generate tokens for any custom date formats you have configured.

I'm posting it up here in case its of any use to anyone. I'm not sure if this is within submission guidelines. I'll post the patch in a moment once I have an nid for the issue...