Overall improvements and fixes

I made several changes both small and more important that I think may be helpful.

• Format and lint code
• Update docs
• Fix bugs and small overlooks. Some were harmless since they were dormant and never executed but others caused faulty tokenization generating broken LaTeX after translation.
• Made Google Translate available no matter the environment variable and made it raise an exception instead if no API key is set
• Made the newly introduced tests in test_translator.py and test_main.py optional with a new --runapi CLI option for pytest. See the related commit 3e7a962c for reasoning. Related to #4.

Feel free to check the changes introduced and the commit messages. These are my proposed changes but we can discuss and change my implementation choices of course.