feat(isPhoneNumber): Add isPhoneNumber validator and tests

[ghost]

Add isPhoneNumber validator in reference to issue #1915.
Create isPhoneNumber.js, and add it to index.js, README.md, and write tests in validators.js.

This is one of my first pull requests so I am still unsure as to what do when it comes to all this. I saw the issue and it seemed like something I could do, so I tried to add a basic implementation. I hope I did not step on anyones toes by taking up this issue. If there are any issues or problems please let me know and I will get on it. Any criticisms appreciated.

Checklist

• PR contains only changes related; no stray files, etc.
• README updated (where applicable)
• Tests written (where applicable)

[codecov]

Codecov Report

Merging #1917 (3a5f6c0) into master (f055c11) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #1917   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          102       103    +1     
  Lines         2072      2079    +7     
  Branches       472       473    +1     
=========================================
+ Hits          2072      2079    +7     

Impacted Files	Coverage Δ
src/index.js	100.00% <100.00%> (ø)
src/lib/isPhoneNumber.js	100.00% <100.00%> (ø)

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[WikiRik]

Back in 2013 the idea was rejected, see #167
I think the same reasoning still applies now.

Also, I don't think this really solves #1915. For that I would add Moldova to isMobilePhone like in #1846 for example

[ghost]

Thanks for the feedback. I will start looking into that.