✨ Adds 'shell' rule for checking cli output via regexp

[skellock]

This adds a new shell rule. For example:

{
  "git email": [{ 
    "rule": "shell",
    "command": "git config user.email", 
    "match": ".+@.+"
  }]
}

I opted out of the expression evaluation since bc can do some (or all?) of it.

I also left off the exit code stuff. I'm not sure if it's best in here or a new exit code rule? Either way, I don't have an immediate need for it. This matching stuff will probably fulfill most of the needs for now.

Closes #133

[codecov[bot]]

Codecov Report

Merging #135 into master will decrease coverage by 0.04%. The diff coverage is 85.18%.

@@            Coverage Diff            @@
##           master    #135      +/-   ##
=========================================
- Coverage   77.44%   77.4%   -0.05%     
=========================================
  Files          24      25       +1     
  Lines         470     500      +30     
  Branches       80      86       +6     
=========================================
+ Hits          364     387      +23     
- Misses         77      82       +5     
- Partials       29      31       +2

Impacted Files	Coverage Δ
src/types.ts	100% <ø> (ø)	:arrow_up:
src/extensions/functions/printResults.ts	100% <ø> (ø)	:arrow_up:
src/commands/report.ts	79.16% <ø> (ø)	:arrow_up:
src/extensions/functions/reviewRule.ts	81.81% <100%> (+1.81%)	:arrow_up:
src/extensions/functions/checkRequirement.ts	75.67% <71.42%> (-4.93%)	:arrow_down:
src/extensions/functions/checkShell.ts	88.88% <88.88%> (ø)

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[skellock]

AppVeyor is failing because of the upstream gluegun issue.

[GantMan]

Adding this new rule to the report functionality!