Using NODE_ENV to determine if frontend code should be included in the build.

Support for palette switching option in libretro

Closes #37

See merge request !39

Made use of const values to store global data.
Polling is done on every tick, supposedly it's the best approach for libretro.

Added more error handling.

Support for Python

See merge request !36

Using pySDL2 and sending them as part of the next frame process.
There's still many TODOs pending for performance improvements.

Also changed `save_image()` method to removed auto format append in file name.

New options that allow fine grain control of how to capture a video.

Also made extension (container) and video encoder separate concepts.
More modularity to the way encoding is performed.

Also made Union more explicit, raises Python3 compatibility.

Makes use of temporary directory.