Add sonar formatter.

[xpol]

Add a sonar formatter that output sonarsource Generic Issue Data format.

[codecov-io]

Codecov Report

Merging #194 into master will increase coverage by 0.01%.
The diff coverage is 98.71%.

@@            Coverage Diff            @@
##           master    #194      +/-   ##
=========================================
+ Coverage   97.38%   97.4%   +0.01%     
=========================================
  Files          52      52              
  Lines        5512    5587      +75     
=========================================
+ Hits         5368    5442      +74     
- Misses        144     145       +1

Impacted Files	Coverage Δ
src/luacheck/format.lua	99.58% <98.71%> (-0.42%)	⬇️

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[alerque]

@xpol If you are still around would you mind opening this branch as a PR against this repository (the new canonical home, see #198). I'll be honest at first pass I'm not ever sure what this PR is about, but the place to get community feedback and if this makes sense to work through getting it tested and merged would be a PR on that repo! Thanks.