fix: only use arrays in results for multiples

[shadowspawn]

In the old parse results, values[option] is always an array whether or not multiple is used. This PR makes it so an array is not used unless the option is listed in multiple, in which case values[option] is an array from the first value added.

Fixes #19

(Code previously proposed and reviewed in #26, so will look familiar logic to some.)

Side note. multiples does not make the values start with an empty array because:

• we don't do defaults for other options
• an empty array would make detecting whether the option has been used different for a multiple

[Eomm]

I did not find a test case using multiple and a single value. Do you think it is worth adding it?

Other than that LGTM

[shadowspawn]

Good suggestion, thanks @Eomm. Will do.