implementation of permutation_opt

[Stargateur]

Not perfect but open a door to Closes #1153

[coveralls]

Coverage increased (+0.003%) to 81.054% when pulling aff0cba on Stargateur:impl_perm_opt into 02433c2 on Geal:main.

[Geal]

I am still not sure if I want permutation_opt included in nom. I wrote one for matroska and that kind of combinator is very error prone

[Stargateur]

This one https://github.com/rust-av/matroska/blob/95a75e389cb7cfa70b847ee09560b0fdeffac0f4/src/permutation.rs ?

There is some difference with my this simple one, seem, matroska one allow mix of optional parser, where this one force all optional or nothing.

I agree we could have this as an external crate as a follow up of #1439, I try to make a crate https://crates.io/crates/nom_permutation, first time for me so probably a lot of mistake.

In parallel, I work on a expand to n! as talk #1448, I believe now it's a external crate the size problem matter less.