Permutation

[sweezy793]

Do the checklist before submitting the PR:

• [x] Have you read the guidelines mentioned in CONTRIBUTING.md?
• [x] Have you squashed your commits?

Q: What version of Node.js you've used in the project? (e.g. Node v10.11.0) A:v10.15.1

Q: Mention the Issue Number! (e.g. Fixed #8) A: Fixed #84

• Give additional information regarding the PR below: Added String Permutation in recursive form along with string validation.

[codecov-io]

Codecov Report

Merging #94 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #94   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          53     55    +2     
  Lines         647    672   +25     
=====================================
+ Hits          647    672   +25

Impacted Files	Coverage Δ
src/index.js	100% <100%> (ø)	:arrow_up:
src/permutation/index.js	100% <100%> (ø)
src/validation/string.js	100% <100%> (ø)

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