Add isPort validator

[frontsideair]

I use it a lot so I wanted to add it.

[chriso]

Thanks for the PR! 😄

[frontsideair]

Thanks for very quick merge!

[jyjin]

I need to use it, too. but I updated to the last version on npm package, it doesn't have method "isPort"

[chriso]

@jyjin:

/tmp $ npm i validator
+ validator@9.0.0
added 1 package in 1.808s
/tmp $ node
> var validator = require('validator')
> validator.isPort('80')
true