v0.6 - massive performance improvements + capture part of the screen

This release marks major performance improvements to the library and adds the feature to capture part of the screen.

Changes to the public API

• The start_recording method of the ScreenRecorder class now accepts one more parameter - the monitor and its dimensions to capture.
• Removed custom exception classes InvalidCodec and InvalidStartMode.

Internal changes

• Switched to using the mss library for capturing screenshots instead of pyscreeze.
• pyscreenrec no longer temporarily writes screenshots to disk, instead it writes images directly to the video stream, saving time when stop_recording is called.
• The image writing work is delegated to a different thread.
• Refactorings.