tslint emit failures as errors

[zinserjan]

Introduces option tslintEmitErrors to override the defined severity for each rule and emit failures always as error.

resolves #27

[codecov-io]

Codecov Report

Merging #32 into master will increase coverage by 0.11%.
The diff coverage is 45.45%.

@@            Coverage Diff             @@
##           master      #32      +/-   ##
==========================================
+ Coverage   40.29%   40.41%   +0.11%     
==========================================
  Files           8        8              
  Lines         335      339       +4     
  Branches       39       44       +5     
==========================================
+ Hits          135      137       +2     
- Misses        200      202       +2

Impacted Files	Coverage Δ
src/worker/TsCheckerRuntime.ts	0% <0%> (ø)	⬆️
src/checker/resultSerializer.ts	50% <0%> (-6.25%)	⬇️
src/TsCheckerWebpackPlugin.ts	96.15% <100%> (+0.1%)	⬆️
src/worker/TsCheckerWorker.ts	90.24% <100%> (ø)	⬆️

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