[Feature proposal] Number assertions

Hi,

I was curious on why there is no implemented assertion for numbers beside `FloatAssertion` which enables precision tolerant assertions. We could have things like `assert_that!(a).is_less_than(b)`, `assert_that(a).is_negative()`, `assert_that(a).is_between(b, c)` etc. I'd be willing to implement some of them for you if you like the idea, but I don't want to invest the time if you do not.

I'm not a big Rust expert (only have one month under my belt), but my thought was that this should be easy to do with macro rules so that we can support all primitive types without much code duplication.