Add log levels

[rfoel]

I thought we can implement a plugin option for controlling log output verbosity.

For example,

• INFO: Most verbose logging output. Prints any informational log (e.g. prints entire layer replacements without deduplication, with entire context including function name and layer arn)
• WARN: Prints warnings only. Prints warnings only (e.g. prints deduplicated layer replacements - which current PR does)
• ERROR: Prints errors only. Prints errors only (e.g. unknown layer arn)

Then, Users can control log verbosity like this:

// serverless.yml

// ... (truncated)
plugins:
  - serverless-latest-layer-version
custom:
  latestLayerVersion:
    logLevel: WARN

Anyway this isn't mandatory for merging. You can ignore this request ;)

Originally posted by @mooyoul in #33 (comment)