Add Repository Groups (or Repository Tags)

[Chris1320]

Add groups/tags to allow the user to backup a group of restic repositories at once. For example:

# Create a new resteasy vault
resteasy init

# Create two groups
resteasy group new stuffA --description "Repos that are related to each other"
resteasy group new stuffB

# Create new repositories with group associations
resteasy add foo --group stuffA
resteasy add bar --group stuffA,stuffB  # repos can join multiple groups

# Add existing repositories to groups
resteasy add baz
resteasy group add baz stuffB

resteasy backup stuffA  # This will back up foo and bar
resteasy backup stuffB  # This will back up bar and baz

[Chris1320]

We could also automatically create groups when they are added to the repositories and remove them when no repositories are on the group, so that we won't need the group command.