Merge multi-value options properly when extending

[aanand]

Currently, environment and volumes are merged when a service extends another. With other multi-value options, however, no merging is done - instead, the new list completely replaces the old. The following need to be fixed:

• external_links
• ports
• expose
• labels (do this in WIP: labels option in docker-compose.yml #1139)
• dns
• dns_search

[marun]

#dibs