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
43561563 (43561563)
but I don't think that the magic is worth introducing ATM.

Closes #1236

include/internal/catch_matchers_generic.cpp

@@ -0,0 +1,9 @@
+#include "catch_matchers_generic.hpp"
+
+std::string Catch::Matchers::Generic::Detail::finalizeDescription(const std::string& desc) {
+    if (desc.empty()) {
+        return "matches undescribed predicate";
+    } else {
+        return "matches predicate: \"" + desc + '"';
+    }
+}