Custom presets

[FeBe95]

Is your feature request related to a problem? Please describe.
Currently only pre-installed presets are supported. Switches for custom presets are not possible.

Describe the solution you'd like
Something like this would be nice:

"presetSwitches": [
    ...
    {
        "name": "Red & Purple Fading",
        "ips": {"192.168.1.101": "255,255,255"},
        "preset": "custom",
        "presetMode": "gradual",
        "presetString": "26,0,0 0,0,0 26,0,26 0,0,0",
        "speed": 57
    },
    ...
]

resulting in a call to flux_led with the following parameters:

flux_led.py 192.168.1.101 -C=gradual 57 "26,0,0 0,0,0 26,0,26 0,0,0"