Fix and improve Streamlink code

Move streamlink code to its own file

Fixes #275

Untested on linux, but should work decently there as well.

src/util/streamlink.hpp

@@ -0,0 +1,27 @@
+#pragma once
+
+#include <QString>
+
+#include <stdexcept>
+#include <string>
+
+namespace chatterino {
+namespace streamlink {
+
+class Exception : public std::runtime_error
+{
+public:
+    using std::runtime_error::runtime_error;
+};
+
+// Open streamlink for given channel, quality and extra arguments
+// the "Additional arguments" are fetched and added at the beginning of the streamlink call
+void OpenStreamlink(const QString &channelURL, const QString &quality,
+                    QStringList extraArguments = QStringList());
+
+// Start opening streamlink for the given channel, reading settings like quality from settings
+// and opening a quality dialog if the quality is "Choose"
+void Start(const QString &channel);
+
+}  // namespace streamlink
+}  // namespace chatterino