Add quick sort (functional variant)

[mik-laj]

Hi my friend.

What do you think about it? Javascript is not just a classic language, but it already has functional programming elements. This allows you to express some problems in a clearer way.

Thanks in advance.

[mgechev]

Added comments. I can merge once you address them. Thanks for the PR!

[mgechev]

@mik-laj added two more comments.

[mgechev]

@mik-laj would you fix the errors and make sure your build passes (gulp build).

[mik-laj]

@mgechev I think the tests are not exactly accurate.

File: https://github.com/mgechev/javascript-algorithms/blob/master/test/sorting/sort.testcase.js#L25 define 4 test cases, but some are checking the result of the function, and some old references array.:

• should work with empty array - test function result
• should work with sorted arrays - test function result
• should work with random non-sorted arrays - test old reference
• should sort the numbers in descending order when such comparator is provided - test old reference It is assumed that this behavior should be determined and that the result of the function should always be checked.

[mgechev]

@mik-laj thanks for the PR! Will update the list of contributors in the next a couple of days.