Multiple assertions on panic messages

[florian1345]

It would be nice to apply multiple assertions to the caught message of a panicking function through .panics() or similar functions. Some ideas:

• .panics() and related functions return a special AssertThatPanic which allows fluent assertions on all panic metadata
• a new function .panics_with_message_that() which returns an AssertThat<String> for the message
• a new function .panics_with_message_satisfying(...) that takes an assertion function