Copy Snapshots to different regions

Added `-g` and `-d` parameters and updated documentation.
`-g` is added to a backup to schedule the resulting snapshots for copy later.
`-d` is used with `-s regioncopy` to actually initiate the region copies.

I also modified `prerequisite_check()` to allow for conditional checks to preserve backwards compatibility. ec2-copy-snapshot is relatively new and I didn't want older versions breaking.

Note that my IDE also removed some trailing spaces.

I also considered adding `-l` to limit the copies to a specified quantity and the option to schedule existing snapshots, although neither has been implemented yet.