feat: Make cookie handling optional via cargo feature

[mythmon]

Cookie handling includes a dependency that uses the excellent lalrpop parsing library. Unfortunately, lalrpop is also fairly slow to compile. For projects that don't need cookie handling, it's wasted compile time. Making cookie handling in this crate optional helps speed up downstream compile times.

I tested this using cargo-hack:

$ cargo-hack --feature-powerset test

[alexliesenfeld]

Thanks for providing this PR. Looks good! This will contribute to reducing dependencies as discussed in #46 as well.