Remove NSParameterAssert() for color
Remove NSParameterAssert() for color

It's legal for it to be nil now.

nuudles/CocoaMarkdown,blackfog/CocoaMarkdown,nuudles/CocoaMarkdown,X8/CocoaMarkdown,cnbin/CocoaMarkdown,cnbin/CocoaMarkdown,indragiek/CocoaMarkdown,blackfog/CocoaMarkdown,dezinezync/CocoaMarkdown,neonichu/CocoaMarkdown,X8/CocoaMarkdown,X8/CocoaMarkdown,dezinezync/CocoaMarkdown,indragiek/CocoaMarkdown,indragiek/CocoaMarkdown,neonichu/CocoaMarkdown