Remove log level

[notmandatory]

Description

This issue fixes #1 by removing the --log_level option which was overriding the RUST_LOG environment variable but does not provide the same level of logging control.

Notes to the reviewers

Also in this PR I added a couple more usage examples in the README, including an example for using the RUST_LOG env variable.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[notmandatory]

@RCasatta I moved #1 to the new bdk-cli repo and this is the fix for it.