Replace : with _ in default file format

[steveyuroff]

While having hh:mm:ss in the date/timestamp is logical, it makes a default incompatibility for those who sync their Google Drive to a Synology NAS for further redundancy and backups. Files with a : in them do not sync.

This is easily solved through the GUI by tweaking the backup name template, but someone not looking closely enough might not realize they're getting less redundancy than they would expect.