Add NSUnderlineColorAttributeName support.

Author: natestedman

README.md

@@ -50,7 +50,7 @@ The provided attribute functions take the value of the attribute as the first va
 #### Framework Types
 The framework types extended with attribute functions are:
 
-- `NSColor`/`UIColor`: `foregroundAttribute`, which maps to `NSForegroundColorAttributeName`, and `backgroundAttribute`, which maps to `NSBackgroundColorAttributeName`
+- `NSColor`/`UIColor`: `foregroundAttribute`, which maps to `NSForegroundColorAttributeName`; `backgroundAttribute`, which maps to `NSBackgroundColorAttributeName`; and `underlineAttribute`, which maps to `NSUnderlineColorAttributeName`.
 - `NSFont`/`UIFont`: `attribute`, which maps to `NSFontAttributeName`.
 - `NSTextAttachment`: `attribute`, which maps to `NSAttachmentAttributeName`.
 - `NSParagraphStyle`: `attribute`, which maps to `NSParagraphStyleAttributeName`.