Autodetect endianness on export

[albfan]

fixes #15

[glatosinski]

Hello,

Thank you for your contribution. Ability to configure endianness during export is indeed a useful feature.

First things first, I'd fix the formatting of the code so that the CI will pass - there is one issue reported by yapf.

You can run code formatting with:

./.github/check_code_formatting.sh

[albfan]

Reimplemented.

Test with:

pic803.zip

raviewer -f pic803.raw -e . --endianness little
raviewer -f pic803.raw -e . --endianness big
raviewer -f pic803.raw -e . --endianness auto

There's only a side effect for this. --endianness might be seen as a set for opening a file, but right now is only effective at --export

[glatosinski]

I introduced handling --endianness in GUI, thanks, merged