Logging level config with RUST_LOG env variable

[austinabell]

Summary of changes
Changes introduced in this pull request:

• Enables env filter configuration with our logger
  • Was previously statically set, but using the underlying builder type we can achieve what we actually want with Logging level flag #237
  • Can use with RUST_LOG=trace ... and more configurable options following https://doc.rust-lang.org/1.1.0/log/index.html with something useful like: RUST_LOG="info,chain_sync::sync=trace" to trace specific modules

Reference issue to close (if applicable)

Closes #237

Other information and links

[ec2]

nice