Add escape hatches

[ncfavier]

Adds a few missing formats and extensions, and, most importantly, generic ways to use formats, extensions and command-line arguments not yet supported by the crate.

[oli-obk]

I think we should still keep the enum nonexhaustive, so we can add more explicit variants later

[ncfavier]

Fair

[oli-obk]

Thanks!

[ncfavier]

Thanks! If you could publish a new release, that would be awesome

[oli-obk]

Yea, i'll do that likely tomorrow. Can't do it from my phone ^^

[oli-obk]

Published as 0.8.10

[ncfavier]

Thanks!