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

projects/SelfTest/UsageTests/Matchers.tests.cpp

@@ -32,6 +32,9 @@ namespace { namespace MatchersTests {
         return "some completely different text that contains one common word";
     }
 
+    inline bool alwaysTrue(int) { return true; }
+    inline bool alwaysFalse(int) { return false; }
+
 
 #ifdef _MSC_VER
 #pragma warning(disable:4702) // Unreachable code -- MSVC 19 (VS 2015) sees right through the indirection
@@ -396,6 +399,26 @@ namespace { namespace MatchersTests {
             }
         }
 
+        TEST_CASE("Arbitrary predicate matcher", "[matchers][generic]") {
+            SECTION("Function pointer") {
+                REQUIRE_THAT(1,  Predicate<int>(alwaysTrue, "always true"));
+                REQUIRE_THAT(1, !Predicate<int>(alwaysFalse, "always false"));
+            }
+            SECTION("Lambdas + different type") {
+                REQUIRE_THAT("Hello olleH",
+                             Predicate<std::string>(
+                                 [] (std::string const& str) -> bool { return str.front() == str.back(); },
+                                 "First and last character should be equal")
+                );
+
+                REQUIRE_THAT("This wouldn't pass",
+                             !Predicate<std::string>(
+                                 [] (std::string const& str) -> bool { return str.front() == str.back(); }
+                             )
+                );
+            }
+        }
+
 } } // namespace MatchersTests
 
 #endif // CATCH_CONFIG_DISABLE_MATCHERS