`auto` mode for the upcoming `scheme` option

[heidrifx]

Like briefly discussed in #99, I suggest adding scheme = "auto" to the upcoming auto-generated themes. This would then choose either light or dark, depending on the relative luminance of the wallpaper.

In short, to get the appropriate relative luminance for a wallpaper, the following steps have to be taken:

1. Get the average color of the image
2. Calculate the relative luminance using the following formula: Y = 0.2126 * R + 0.7152 * G + 0.0722 * B (see link above)
3. Decide on the scheme based on the resulting relative luminance Y (<= 0.5 could be dark, else light)

The threshold could be an optional field for the user. The suggested auto option should be explained in the docs/wiki.

[prankstr]

Ahhh right, completely forgot about that. I wonder if scheme = "auth" is need at all or if its enough that it's auto by simply omitting it, and you only explictly set dark or light of you don't want it to be automatically derived from relative luminance, i.e mode = "auto" is truly auto unless you override scheme. Does that make sense?