Add options.string

[jorgebucaran]

Add a new string option to string type coerce flag/options.

getopts(["-vcv"], { string: ["c"] }) //=> { v: true, c: "v" }

For comparison, here is the default behavior without { string: ["c"] }

getopts(["-vcv"]) //=> { v: [true, true], c: true }

Implemented properly this should only slow us down ever so slightly.