Add dub fetch <package>@<version> shortcut

[wilzbach]

Follow-up to #1428 (comment)

FWIW the recently added dub add uses =, but as both = and : can't be part
of a valid version we can just support both as valid splitting symbols to avoid any potential confusion.

edit: now changed to @ instead of :

[dlang-bot]

Thanks for your pull request, @wilzbach!

[s-ludwig]

: is used as the sub package delimiter, so that may lead to confusion (while I think the set of valid package names is distinct from the set of valid version names). I'd suggest to use the more commonly found @ instead.

[wilzbach]

I'd suggest to use the more commonly found @ instead.

Fair point. Changed to @.