Add `String.doesNotContain`

[StefMa]

I jsut realized an assertion on String.contains("") exist. I think it is usefull to add also an String.doesNotContain("").

expectThat("Something").contains("S") // true
expectThat("Something").doesNotContain("S") // false

[dump247]

You should be able to do .not().contains("string") or .not { contains("string") }