Using linter package golangci-lint, make it a module

[dertseha]

Description

This PR adds an initial configuration for the linter package golangci-lint and adherence to the linters. Some linters are disabled, as their adherence would need larger refactoring (possibly API change), most notably gosec/errcheck. Furthermore a go.mod file is added, as go modules are now common.

The solution

• Add .golangci.yml file, which is the configuration for golangci-lint (they also have a free service to run the linter online)
• enable all the linters and their messages
• ignore linter with too many results / need for clarification
• Handling of "small" issues

Points of possible API changes

• exported variable SkipThisCombination made to a constant. I don't think it is intended to be modifyable by others.
• function NewPrintSupportedDiffProgramsReporter() returns the probably (?) intended type.

Notation

I wasn't familiar with Arlo's git notation and will probably need some time until I am. Still, the commits are small steps.

[codecov-io]

Codecov Report

Merging #9 into master will decrease coverage by 0.41%. The diff coverage is 10.52%.

@@            Coverage Diff             @@
##           master       #9      +/-   ##
==========================================
- Coverage   48.14%   47.73%   -0.42%     
==========================================
  Files          16       16              
  Lines         459      463       +4     
==========================================
  Hits          221      221              
- Misses        230      234       +4     
  Partials        8        8

Impacted Files	Coverage Δ
approvals.go	93.97% <ø> (ø)	:arrow_up:
reporters/quiet.go	0% <ø> (ø)	:arrow_up:
reporters/file_launcher.go	0% <0%> (ø)	:arrow_up:
reporters/vscode.go	0% <0%> (ø)	:arrow_up:
reporters/clipboard.go	0% <0%> (ø)	:arrow_up:
reporters/beyond_compare.go	0% <0%> (ø)	:arrow_up:
reporters/intellij.go	0% <0%> (ø)	:arrow_up:
reporters/newbie.go	0% <0%> (ø)	:arrow_up:
reporters/gogland.go	0% <0%> (ø)	:arrow_up:
approval_name.go	80.7% <100%> (ø)	:arrow_up:
... and 1 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 6ae1ec6...d5be98e. Read the comment docs.