Add support for headers for notification callbacks

You can define the DJANGO_VIDEO_ENCODER_NOTIFICATIONS_HEADERS dict in the settings

New video format definition including django checks

• Stable model definition avoiding permanent migrations, fixing #5
• Checks to ensure format consistency

Updated settings and corresponding choices migration

1.0.1

Version bump

First PyPi release after renaming

1.0.0

fix: README now respects rst standards

Pre-rename release

0.10.2

chore: Disable black format check for embedded SQL