Add decimal and negative support for GCD

[Brendonovich]

Adds support for M.gcd() to evaluate the gcd of numbers with decimals and numbers less than 0.

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: Node v12.4.0

Q: Mention the Issue Number! (e.g. Fixed #8) A: Fixed #215 and added extra feature

• Give additional information regarding the PR below: Implements feature requested in #215, as well as adding support to return gcd of negative numbers, returning the gcd as a positive number.

[codecov-io]

Codecov Report

Merging #216 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #216   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          80     80           
  Lines        1451   1463   +12     
=====================================
+ Hits         1451   1463   +12

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

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