Compatible with log4rs

[zonyitoo]

There are still many libraries using the log facilities to emit debug informations, tracing-subscriber could also register a handler to log and output together.

This library aims to share the same configuration process like log4rs, so I suggest:

1. trace4rs could also accept exactly the same configuration format like log4rs, which means:
   • They should have the same configuration fields
   • They should have the same configuration options
   • Format should have the same keys
2. trace4rs could also handle outputs from log, which means:
   • trace4rs could work just like log4rs