cmd/backup: able to specify archive name (#4320)
cmd/backup: able to specify archive name (#4320)

Added flag '--archive-name' for backup command.

pikacloud/pikacode,ForxaCoLab/forxa,mukaiu/gogs,chriswatt/gogs,cgars/gogs,wangjild/gogs,psaavedra/gogs,linquize/gogs,dancewing/gogs,cgars/gogs,dancewing/gogs,psaavedra/gogs,mukaiu/gogs,dancewing/gogs,gogits/gogs,mukaiu/gogs,ForxaCoLab/forxa,pikacloud/pikacode,xaionaro/gogs,xaionaro/gogs,gogits/gogs,chriswatt/gogs,linquize/gogs,psaavedra/gogs,ForxaCoLab/forxa,wangjild/gogs,gogits/gogs,linquize/gogs,wangjild/gogs,dancewing/gogs,cgars/gogs,pikacloud/pikacode,xaionaro/gogs,chriswatt/gogs