Allow creating custom tags

[PMunch]

Currently you can do things like choosenim stable or choosenim devel. But I also like to keep a local copy of the compiler with compiler debug information on, or a feature I'm trying to fix/add. It would be nice if I could "tag" this build so I could do choosenim mydebug instead of choosenim <the debug folder>.

[dom96]

Yeah, this would be a nice feature actually.