Add compression level CLI option

[kakkoyun]

This PR adds compression level CLI option.
Also piggybacks a lot of refactorings. (sorry for lengthy PR)

Fixes #72

Proposed Changes

• Add compression level CLI option
• Logs compression level when debug enabled
• Upgrade go 1.13
• Refactor error messages by using new go errors
• Introduce structured logging
• Refactor Makefile
• Upgrade linter, Golangci-lint
• Automate usage doc creation
• Update docs

Description

Users now can specify compression level.

Checklist

• Read the CONTRIBUTING document.
• Read the CODE OF CONDUCT document.
• Add tests to cover changes.
• Ensure your code follows the code style of this project.
• Ensure CI and all other PR checks are green OR
  • Code compiles correctly.
  • Created tests which fail without the change (if possible).
  • All new and existing tests passed.
• Add your changes to Unreleased section of CHANGELOG.
• Improve and update the README (if necessary).
• Ensure documentation is up-to-date. The same file will be updated in plugin index when your PR is accepted, so it will be available for end-users at http://plugins.drone.io.

[kakkoyun]

@limayekaiwalya It's a long but a good one. Please take your time.

[limayekaiwalya]

@kakkoyun , Looking at it. Sorry for the long response times. I am taking a bit longer to understand the existing code so I can provide better reviews :)

[kakkoyun]

@limayekaiwalya Please tell me if you need anything to understand the existing codebase, happy to help.

[limayekaiwalya]

Looks good!

[limayekaiwalya]

@kakkoyun A questions out of curiosity: Why did we switch to fmt.Errorf from errors.Wrap ?

[kakkoyun]

@limayekaiwalya The real change is about %w in fmt.Errorf which has introduced recently with 1.13 release of Go, now we can wrap standard errors with a context without a need of an external package. Further, since we don't actually utilize all the features from pkg/errors package, like stack capture etc., I thought we don't need any other library for that, hence I removed it.