Add isHarshad() to Mathball

[pydevsg]

Do the checklist before submitting the PR:

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

Q: What version of Node.js you've used in the project? (e.g. Node v10.11.0) A:v11.6.0

Q: Mention the Issue Number! (e.g. Fixed #8) A:Fixed Issue #28

• Give additional information regarding the PR below: Created a function isHarshad() which accepts a positive number and checks whether it is Harshad number or not, in all other cases it shows errors.

[codecov-io]

Codecov Report

Merging #63 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #63   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          42     43    +1     
  Lines         433    442    +9     
=====================================
+ Hits          433    442    +9

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

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