Invert assert

[Riduidel]

As a developer, I sometimes want to have inverted asserts.
Typically, I would like to have a assert_that!(...).not().starts_with(...) to make sure that my string does not starts with a prefix.

[Riduidel]

Maybe it already exists, but I've not found it in the docs (maybe I searched poorly). If it already exists, you should for sure close that ticket ...