Show possible option values

Potential fix for #37.

To do:

* [x] handle short options
* [x] fix spacing issues
* [x] do not color values

There is currently no way to specify what `value` exactly is. It might make sense to use the long option name as the value, so `--target=value` would become `--target=target`. Does that make sense?

Example:

```
    -a value                lorem ipsum
    -b [value]              lorem ipsum
    -c                      lorem ipsum
    -m --mithril=value      lorem ipsum
    -n --nori=[value]       lorem ipsum
    -o --orthanc            lorem ipsum
       --urukhai=value      lorem ipsum
       --valar=[value]      lorem ipsum
       --weathertop         lorem ipsum
```

CC @bmesuere.