feat(variable): extend string-variable

[gregswindle]

Pull Request (PR)

Add operator methods to StringVariable:

1. endsWith
2. hasMaxLength
3. hasMinLength
4. isEmpty
5. isNotEmpty
6. isOneOf
7. includes
8. matches
9. startsWith

Validate arguments to be of an expected type, when
applicable.

Associated issue(s)

Closes #33

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• 1. The acceptance criteria for all associated issues have been
  completed, tested, and validated.
• 2. The README either reflects my changes or does not require updates.
• 3. The CONTRIBUTING document either reflects my changes or does not
  require updates.

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[qlty-cloud-legacy]

Similar blocks of code found in 2 locations. Consider refactoring.

[qlty-cloud-legacy]

Code Climate has analyzed commit f58b844 and detected 6 issues on this pull request.

Here's the issue category breakdown:

Category	Count
Duplication	6

View more on Code Climate.