Add support for custom torrc configurations

[pluja]

Hi, first off, thank you for maintaining this great project!

I needed to use some custom configurations in the torrc, and I thought it would be nice to have an option to directly configure it via environment variables, so I made a few changes to the entrypoint script to implement that. I also made a few other small changes to the script (added functions, etc), please do check it and test it (I have tested it myself, and it works fine for me).

Basically, new env variables are accepted with the format TORRC_[CONFIG] (for globally applied torrc options) or TORRC_[SERVICE_NAME]_[CONFIG] for service-specific configurations. Here's an example:

docker run -d --name tor-hidden-service \
  -e HS_WEB=web-container:80:80 \
  -e HS_API=api-container:8080:80 \
  -e TORRC_MaxCircuitDirtiness=600 \
  -e TORRC_NewCircuitPeriod=30 \
  -e TORRC_WEB_HiddenServiceMaxStreams=10 \
  -e TORRC_API_HiddenServiceMaxStreams=5 \
  ghcr.io/hundehausen/tor-hidden-service:latest

This would apply the first two configurations globally, and the last two to their respective services.

Tell me if you want me to change anything or any question. Feel free to close the PR if it's out of the scope of the project!