Add version of StringAssertions.Contain that allows specifying the number of times a substring should occur. #818
Description
Currently StringAssertions has no easy way of asserting that a substring should occur a particular number of times within a string. Imagine the syntax could be something like
actual.Shoud().Contain("Fred", Twice, "Fred is important");
or
actual.Should().Contain("Fred").Twice("Fred is important");
or
actual.Should().Contain("Fred", 2, "Fred is important");