Truncate a String in Swift 2.0

I’ve been working on a project developing an iOS app in Swift and found the need to trim a string down to a specific number of characters. Since most of my coding experience was with the web, I started searching for the easy to use equivalent of Javascript’s .substring() function. Unfortunately I wasn’t able to find one that was so cut and dry. I did however take what I was able to find and extend the String class to add my own function to it.