Add a function, `util.parseArgs()`, which accepts an array of
arguments and returns a parsed object representation thereof.

Ref: nodejs/tooling#19

- rename `expectsValue` to `optionsWithValue`
- output an object with props `options` and `positionals` instead of magic `_`
- add `multiOptions` option, which results in arrays when parsed
- remove "count" functionality in lieu of `multiOptions`
- update examples in docstring
- update tests
- update docs