Ensure `g:a:v:` and `g:a::m` work as endpoints

[ctrueden]

The x:y:z endpoint triple is ambiguous: is z a version or a main class? (Or a classifier, or...?) To avoid the ambiguity, it would be great if endpoints could be padded with empty strings, so e.g. g:a::m would unambiguously mean g:a:RELEASE:m and g:a:v: would unambiguously mean the same thing as g:a:v (i.e. detect the main class from the JAR manifest) no matter what pattern the v takes, so that jgo does not need to use a regex heuristic to know that a version is what is meant.

[hanslovsky]

I really like this, simple and elegant.