Add PredicateMatcher that takes an arbitrary predicate functions

Also adds `Predicate` helper function to create `PredicateMatcher`.
Because of limitations in type inference it needs to be explicitly
typed, like so
`Predicate<std::string>([](std::string const& str) { ... })`.
It also takes an optional second argument for description of the
predicate.

It is possible to infer the argument with sufficient TMP, see
https://stackoverflow.com/questions/43560492/how-to-extract-lambdas-return-type-and-variadic-parameters-pack-back-from-gener/43561563#43561563
but I don't think that the magic is worth introducing ATM.

Closes #1236

include/internal/catch_capture_matchers.h

@@ -11,6 +11,7 @@
 #include "catch_capture.hpp"
 #include "catch_matchers.h"
 #include "catch_matchers_floating.h"
+#include "catch_matchers_generic.hpp"
 #include "catch_matchers_string.h"
 #include "catch_matchers_vector.h"