Extend ESLint functionality

[Tomtomgo]

• Allow specifying the eslint executable path via the setting GeneralOption.ESLINT_EXECUTABLE. This is useful when running a non-global install of eslint.

• When specifying an alternative folder, the plugins used by ESLint can't be found, as ESLint searches in its working directory. It can search in a different directory, specified by https://eslint.org/ docs/user-guide/command-line-interface#--resolve-plugins-relative-to. So, we also allow configuring this folder through GeneralOption.ESLINT_PLUGINS_FOLDER.

• Do not crash on ESLint's unknown fields in JSON output.

Closes #216.

[codecov-io]

Codecov Report

Merging #217 into master will decrease coverage by 0.19%. The diff coverage is 25%.

@@             Coverage Diff             @@
##             master     #217     +/-   ##
===========================================
- Coverage     73.45%   73.25%   -0.2%     
  Complexity      612      612             
===========================================
  Files           146      146             
  Lines          1955     1963      +8     
  Branches        127      128      +1     
===========================================
+ Hits           1436     1438      +2     
- Misses          462      468      +6     
  Partials         57       57

Impacted Files	Coverage Δ	Complexity Δ
.../touk/sputnik/processor/eslint/ESLintExecutor.java	0% <0%> (ø)	0 <0> (ø)	:arrow_down:
...touk/sputnik/processor/eslint/ESLintProcessor.java	0% <0%> (ø)	0 <0> (ø)	:arrow_down:
...a/pl/touk/sputnik/configuration/GeneralOption.java	98.41% <100%> (+0.05%)	3 <0> (ø)	:arrow_down:
...k/sputnik/processor/eslint/ESLintResultParser.java	73.68% <100%> (ø)	5 <0> (ø)	:arrow_down:

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[SpOOnman]

Looks fine to me! I can release it tomorrow, though.