fix: default should be T[] when multiple: true

[quantumkid]

When using the multiple: true option in a flag the default option should be an array.

For example:

  static flags = {
    foo: flags.string({
      char: 'f',
      description: 'test flag',
      multiple: true,
      default: ['a']
    }),
    bar: flags.integer({
      char: 'b',
      description: 'test flag two',
      multiple: true,
      default: [1, 2, 3]
    })
  }

In this way the type of default matches the type of the flag itself.

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @quantumkid to sign the Salesforce.com Contributor License Agreement.

[codecov]

Codecov Report

Merging #56 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #56   +/-   ##
=======================================
  Coverage   91.35%   91.35%           
=======================================
  Files          11       11           
  Lines         324      324           
  Branches       85       85           
=======================================
  Hits          296      296           
  Misses         12       12           
  Partials       16       16

Impacted Files	Coverage Δ
src/flags.ts	100% <ø> (ø)	⬆️

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[RasPhilCo]

🏄 good catch!