Handle non-unique snapshot names

[d0ugal]

As commented in #37. If you use "filename": "snapshot_name" and you create snapshots without a unique name things get a bit weird. (i.e. only calling the snapshot "snapshot" or creating more than one a day only with the date.

It means that the addon will keep trying to replace the snapshot, as it will find a file with the same name but different contents. I can see a couple of options:

1. Look at all the snapshot names and report an error if the names are not unique and the snapshot_name strategy is used.
2. Add the slug or date to the end of the snapshot name. This could look a little ugly, but it would be easiest.
3. Provide a new naming strategy which is "snapshot_date", this will save the snapshots as "Snapshot 2018-11-15 08:34:02" and ignore any name given in Hass.io This would then lose any useful information added in the snapshot name.

The easiest solution is for users to create a unique snapshot name. This is trivial with something like this, which will work unless you make more than one snapshot a second (then you probably have bigger issues!).

name: Automated Backup {{ now().strftime('%Y-%m-%d %H-%M-%S') }}

I'm therefore inclined to update the example in the README to add the time and go with option 1 and display an error and quit if the names are not unique (and the snapshot_name option is used). My only concern here is the repeated support requests as this trips up new users in the future.