Add fill-color option

[medusalix]

I added support for the fillColor property to the API and made it accessible through the CLI-option --fill. Setting the background color without an image URL is not possible, though I looked into the way the system prefpane allows you to choose a custom color. Apparently it stores a transparent picture in the bundles' resources: /System/Library/PreferencePanes/DesktopScreenEffectsPref.prefPane/Contents/Resources/DesktopPictures.prefPane/Contents/Resources/Transparent.tiff. It might be possible to use this image to add the additional functionality of setting a solid color background to the CLI.

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[medusalix]

@sindresorhus Any thoughts?

[sindresorhus]

This looks good for setting a fill color with an image, but there needs to be a way to set just a color too as solid background, per the #19. We can use the transparent image trick.

For the API, it could just be an overload of set:

try Wallpaper.set(
	NSColor.systemRed,
	screen: convertStringToScreen(screen.value)
)

(Without scale and fillColor parameters)

As for the CLI, I'm not sure how to model this. Suggestions welcome.

[sindresorhus]

@medusalix Should I close this?

[medusalix]

@sindresorhus I'm still interested in finishing this (as soon as I'm back on my macOS machine).

[medusalix]

@sindresorhus I've corrected the usage in the README and renamed the flag to --fill-color. I've started working on the solid color functionality, but this PR needs to be merged first.