feat: Progressive linting

[umar-muneer]

This PR is dependent on #77

Right now after old and new stats have been collected, they are checked against only a single condition whether the warnings have been increased or not. This PR introduces the concept of guards. A guard is just a condition against which rule stats are checked. There are two guards provided by esplint right now.

1. warningsIncreased Have warnings per file per rule increased than the previous calculated value
2. warningsReducedBy Have warnings been reduced by the specified value. This is very important because developers could be enforced to remove warnings iteratively, in every sprint or a cycle.

By default, the warningsIncreased guard will run. Other guards can be specified through the command line by using the following option: --guards=warningsReducedBy. The following command will enable the guard.

./index.js example/index.js --guards=warningsReducedBy

A new parameter has been introduced in .esplintrc called reduceWarningsBy. This value will be provided in the numerical equivalent of percentages. For example, to enforce reduction by 10% the value specified in the config will be 0.1

[codecov]

Codecov Report

Merging #78 into master will increase coverage by 0.02%.
The diff coverage is 99.36%.

@@            Coverage Diff             @@
##           master      #78      +/-   ##
==========================================
+ Coverage   98.31%   98.34%   +0.02%     
==========================================
  Files          11       16       +5     
  Lines         357      422      +65     
  Branches       47       66      +19     
==========================================
+ Hits          351      415      +64     
- Misses          6        7       +1

Impacted Files	Coverage Δ
lib/config.js	100% <100%> (ø)	⬆️
...g/fixtures/reduceWarningsBy-negative/.esplintrc.js	100% <100%> (ø)
__tests__/config/fixtures/.esplintrc.js	100% <100%> (ø)
lib/fileSet.js	100% <100%> (ø)	⬆️
lib/cli.js	94.02% <100%> (ø)	⬆️
lib/record.js	100% <100%> (ø)	⬆️
...ures/reduceWarningsBy-greater-than-1/.esplintrc.js	100% <100%> (ø)
...s__/config/fixtures/reduceWarningsBy/.esplintrc.js	100% <100%> (ø)
...config/fixtures/reduceWarningsBy-NAN/.esplintrc.js	100% <100%> (ø)
lib/engine.js	98.8% <98.8%> (-1.2%)	⬇️
... and 5 more

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