Replace the trace config file option with a more flexible trace_filter
and expose the same via the ARTI_LOG environment variable.
If ARTI_LOG is set it will override the setting in the config file.

Example:

ARTI_LOG="info,tor_proto::channel=trace" ./target/debug/arti

The syntax is described here
https://docs.rs/tracing-subscriber/0.2.20/tracing_subscriber/filter/struct.EnvFilter.html#directives

Mainly, drop the suggestion to fetch submodules, and explain what
more things do.

(The only submodule we have now is the fuzzing corpora, which most
people won't want.)

(I am not 100% convinced that we should keep 100% of the
instructions for working with git in the long term.  Do many other
projects have these?  Is there a tutorial we should link to
instead?)

Closes #151.

Closes #106