Fix values with = being accepted and ignored for short flag options (#13)

Fixes a bug where short boolean flags allowed values, which were then
ignored (e.g. `-f=value` where `-f` is a flag/boolean).

Author: zefir-git

src/cmd/flag.d

@@ -104,7 +104,7 @@ public class Flag {
             return false;
         return
             (query.startsWith("--") && longName == query[2..$])
-            || (query.startsWith("-") && shortName == query[1..2])
+            || (query.length == 2 && query.startsWith("-") && shortName == query[1..2])
             || query == longName
             || query == shortName;
     }