on above code, multi-router-view make only one instance when a router push both '/c' and '/d'.
but sometimes two different path '/c' and '/d' want two different instances.
Therefore, multi-router-view should have alternative way to set a cache key to handle various business logic.
Added: alt-key property
As mentioned on #9
A major problem of setting key as parent's path is that can't handle default url like *.
on above code, multi-router-view make only one instance when a router push both '/c' and '/d'. but sometimes two different path '/c' and '/d' want two different instances.
Therefore, multi-router-view should have alternative way to set a cache key to handle various business logic.
Usage: