Adding option to enable Tx/Rx LEDs and modify max USB current descriptor

[cr1tbit]

Ok, so I've added some features I needed for my project:

• Tx/Rx LEDs - pretty straightforward - modifying a single byte does the job
• modifying "bMaxPower" USB descriptor - this is also done by modifying a single byte, however it's done against the documentation, as described in new README.md - therefore I decided to hide it behind an -x flag.
• I also did some testing with "Battery Charging" options - and realized that the feature is borderline useless - so I added an paragraph at the bottom of the README.md, so that people won't waste their time trying to get it to work.

I wonder, if at this point, this project shouldn't become cp2102n - specific.

[ladyada]

bless 🙏 this works a treat!