Custom separator between `ARG` and help message

[wleoncio]

I read through #82 and would like to propose a—hopefully—simple version of it.

Hopefully it's straightforward enough to allow the user to customize the separator between an argument and its help message? The default (: ) misaligns everything, making the docs harder to read, so it would be great help to be able to set it to \t\t or whatever. No worries about line-wrapping or anything else mentioned in #82 (those strings shouldn't be long anyway).

As a workaround, I manually add spaces to the help message inside the ARG_*() call, but I see it doesn't parse \t and there's no way to get rid of the colon (arguably unnecessary if arg and description are properly spaced).

Thanks for making this very useful tool, BTW! :)