Xcode 9 asset catalog color set #2

Relocated from nordtheme/nord#45
Requested by @ornithocoder

Xcode 9 added color sets to asset catalogs. It can be used in the Interface Builder and programmatically via UIColor(named:).

It should be added as official Nord asset catalog that can be dragged and dropped into projects.

Blog Entries:

• Asset Catalog Changes in Xcode 9
• Asset catalog colors on Xcode 9