Support selecting snapper's config, default to root

[maximbaz]

The current code cannot properly handle the case when multiple snapper configs exists. The IDs collide, and the description is set incorrectly.

However, we don't actually need to support multiple configs, we need only the "root" config which is where the OS resides.

This PR adds the option to select the config name, and defaults to root because that's how most people name their snapper config for the /.

/cc @ckotte

Fixes #40

[ckotte]

I tested the fix on my system with three subvolumes and now the description is properly set.

[ckotte]

Thanks!

[maximbaz]

@Antynea gave me contributor access to this project, since it's tested I'll go ahead with merging and making a new release because I think this is a quite important bug fix. If you have comments on this PR, we can address them afterwards.