Update SparkleExtensions.cs

Hi! Maybe you can use StringBuilder in the Combine method. Strings are inmutable, then when you reassign a new value, you are making copies of the string, and wasting some memory, maybe not much in this case, but imagine that parts have 10000 elements (maybe impossible, i know), you are going to make 10000 copies. With StringBuilder it doesn´t happend! 
Best regards! 
I hope you can use it