Support flow-router groups

[zimme]

ActiveRoute.group and {{isActiveGroup}}.

Can we translate this over to iron:router too, by checking prefix in url?

[francisbyrne]

+1

[slissner]

+1

[thearabbit]

+1, Now could I use it with group?

[thearabbit]

but I try to use isActivePath instead.

[thearabbit]

I tried to use isActivePath, but it don't reactive when I change link.