std::chrono overloads for .timeout() and .delay() (#30)

Add convenience overloads accepting durations from the C++ Standard Library.

docs/qtpromise/qpromise/delay.md

@@ -20,3 +20,31 @@ auto output = input.delay(2000).then([](int res) {
     // called 2 seconds after `input` is fulfilled
 });
 ```
+
+---
+
+*Since: 0.6.0*
+
+```cpp
+QPromise<T>::delay(std::chrono::milliseconds msec) -> QPromise<T>
+```
+
+This is a convenience overload accepting [durations from the C++ Standard Library](https://en.cppreference.com/w/cpp/chrono/duration).
+
+```cpp
+QPromise<int> input = {...}
+auto output = input.delay(std::chrono::seconds{2}).then([](int res) {
+    // called 2 seconds after `input` is fulfilled
+});
+```
+
+C++14 alternative:
+
+```cpp
+using namespace std::chrono_literals;
+
+QPromise<int> input = {...}
+auto output = input.delay(2s).then([](int res) {
+    // called 2 seconds after `input` is fulfilled
+});
+```

docs/qtpromise/qpromise/timeout.md

@@ -25,3 +25,39 @@ auto output = input.timeout(2000)
         // operation timed out!
     });
 ```
+
+---
+
+*Since: 0.6.0*
+
+```cpp
+QPromise<T>::timeout(std::chrono::milliseconds msec, any error = QPromiseTimeoutException) -> QPromise<T>
+```
+
+This is a convenience overload accepting [durations from the C++ Standard Library](https://en.cppreference.com/w/cpp/chrono/duration).
+
+```cpp
+QPromise<int> input = {...}
+auto output = input.timeout(std::chrono::seconds{2})
+    .then([](int res) {
+        // operation succeeded within 2 seconds
+    })
+    .fail([](const QPromiseTimeoutException& error) {
+        // operation timed out!
+    });
+```
+
+C++14 alternative:
+
+```cpp
+using namespace std::chrono_literals; 
+
+QPromise<int> input = {...}
+auto output = input.timeout(2s)
+    .then([](int res) {
+        // operation succeeded within 2 seconds
+    })
+    .fail([](const QPromiseTimeoutException& error) {
+        // operation timed out!
+    });
+```