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.

v0.5 - bringing the project back to life

I abandoned the project in 2021 because I thought it had no future because of issue #3 , which highlights a fundamental problem with this library. But thanks to @damies13's suggested fix, I managed to raise the frame count by almost twice. With this newfound enthusiasm, I'll continue to maintain this library, and try to make a meaningful contribution to the python ecosystem.