Add `pretty_assertion` cargo feature

[oknozor]

The pretty assertion crate is helpful when dealing with large text assertions.

The current assert_that implementation panic on failed assertions so it's not compatible with pretty_assertion out of the box.

Maybe we could tweak assert_that so it uses assert_eq instead of panic so we can add pretty_assertions as a cargo feature.

[ms-ati]

Ha I came here to ask if we could have the String equality assertions use pretty_assertions !