feat: coerce full array instead of each element

[nexdrew]

Fixes #50.

As a bonus, this also means coercion can apply to argv._. Here's an example:

var parse = require('yargs-parser')
var argv = parse(process.argv.slice(2), {
  coerce: {
    _: function (arg) {
      // do something special here
      return arg
    }
  }
})
console.log(argv)

This works with an explicit string: ['_'] option too (the coerce function would be guaranteed to be given an array of strings).

Note that coercion for implicit arrays (options parsed with multiple values but without array config) still happens per element.