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"
 

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 + '"';
+    }
+}

include/internal/catch_matchers_generic.hpp

@@ -0,0 +1,58 @@
+/*
+ *  Created by Martin Hořeňovský on 03/04/2017.
+ *
+ * Distributed under the Boost Software License, Version 1.0. (See accompanying
+ * file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+ */
+#ifndef TWOBLUECUBES_CATCH_MATCHERS_GENERIC_HPP_INCLUDED
+#define TWOBLUECUBES_CATCH_MATCHERS_GENERIC_HPP_INCLUDED
+
+#include "catch_common.h"
+#include "catch_matchers.h"
+
+#include <functional>
+#include <string>
+
+namespace Catch {
+namespace Matchers {
+namespace Generic {
+
+namespace Detail {
+    std::string finalizeDescription(const std::string& desc);
+}
+
+template <typename T>
+class PredicateMatcher : public MatcherBase<T> {
+    std::function<bool(T const&)> m_predicate;
+    std::string m_description;
+public:
+
+    PredicateMatcher(std::function<bool(T const&)> const& elem, std::string const& descr)
+        :m_predicate(std::move(elem)),
+        m_description(Detail::finalizeDescription(descr))
+    {}
+
+    bool match( T const& item ) const override {
+        return m_predicate(item);
+    }
+
+    std::string describe() const override {
+        return m_description;
+    }
+};
+
+} // namespace Generic
+
+    // The following functions create the actual matcher objects.
+    // The user has to explicitly specify type to the function, because
+    // infering std::function<bool(T const&)> is hard (but possible) and
+    // requires a lot of TMP.
+    template<typename T>
+    Generic::PredicateMatcher<T> Predicate(std::function<bool(T const&)> const& predicate, std::string const& description = "") {
+        return Generic::PredicateMatcher<T>(predicate, description);
+    }
+
+} // namespace Matchers
+} // namespace Catch
+
+#endif // TWOBLUECUBES_CATCH_MATCHERS_GENERIC_HPP_INCLUDED