Add more granular control of target minimum free space

[Corionis]

The current implementation of minimum free space is based on the library. However, a library can span multiple storage devices.

Consider changing targets handling for a better approach.

One idea is to add a Devices section something like:

"devices": [
    {
        "device": "/home/plex/Plex/Media/hd01",
        "minimum": 50GB
    },
    {
        "device": "/home/plex/Plex/Media/hd02",
        "minimum": 100GB
    }
]

[Corionis]

Completed for version 3.00