Group mrouter interfaces by vlan at `sh ip igmp snooping mrouter`.

- see issue #2
- note: temporal complexity is not satisfactory:
((max_vlan - min_vlan) * mrouters.size); an alternative, would be to create a
hashtable when traversing the mrouters list with vlan as key and only then
print the mrouter interfaces grouped by vlan_id.

Signed-off-by: Andreea Hodea <hodea_andreea@yahoo.com>