Support space-seperated short option forms

[tecosaur]

At the moment, if I have an option say

-e, --example <int>: this is an example

I can supply this option four ways:

julia demo.jl -- -e1
julia demo.jl -- -e=1
julia demo.jl -- --example 1
julia demo.jl -- --example=1

There's a fifth standard way that seems to have been overlooked:

julia demo.jl -- -e 1

It would be nice if Comonicon could support this.

[Roger-luo]

julia demo.jl -- -e 1

hmm, I think this is probably a bug of the dash separator, we should support this syntax for the sake of consistency with long options.