Constructor OptionAttribute(char, string) although property ShortName is string

The current constructor of ```OptionAttribute``` only allows for 1 character shortName, although the property ```ShortName``` itself is of type ```string```. Which prevents us from creating options with more than 1 character, though that might be useful in order to make the short name parameter name of similar options _(more)_ memorable for the user, e.g.

```
"fn", "filename"
"fs", "filesize"
```