Added AttributedLabelSegment to allow NSAttributedString segments.

[gregggreg]

For my needs I decided to add a new AttributedLabelSegment which can be used to set the attributedText on the label. This allows you to use much more than just strings or icons in each segment.

For instance, using NSTextAttachment you can create segments that have both an image and a string. Or you can have underline, strikeout text, etc. as well.

Would be great to have this merged in so that I don't have to use my fork long term, but that is up to you if you think this is generally useful.