peripherals/eos-evm-ws-proxy: make logging configurable

[cc32d9]

logger.js has a hardcoded choice for logfiles in the current working directory (error.log and combined.log), which mixes up the code with data, and makes log rotation an additional burden.

As I run services under systemd, I prefer stterr or stdout as logging mechanisms. Others may run it under pm2 and prefer stderr as well.

The logger needs to be flexible, and by default it should output to console.