Add zstd support

[MmAaXx500]

Fixes #156

Proposed Changes

• Add zstd as supported compression algorithm

Description

This PR introduces zstd compression algorithm with configurable compression level.

I had to pin the klauspost/compress lib to v1.13.5 because go mod tidy complains about missing io/fs and testing/fstest (see) which only supported on go 1.16+ (we are on 1.14 currently)

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]

LGTM 🥇 🚀

[kakkoyun]

I don't see any reasons not to merge it @jimsheldon