Add option to add recording buffer to beginning of new recordings

[Raging_Flames]

I've switched from MSI Afterburner to OBS as my primary way to record my screen. One thing I liked about MSI that I haven't found in OBS yet is when you are using the replay buffer, MSI automatically adds the buffer to the beginning of any new recording that you start. That way if, say you are playing a game, and something cool happens, you can start recording and have the last X seconds/minutes of game play AND what ever you record after that. This way you don't have to push your "save buffer" hotkey and then the "Start recording" hotkey, and drop some frames in the process.