Perl style substring for Swift String

String+substr.swift

extension String {
    /**
     Perl style substring

     - parameter offset: Negative offset starts that far back from the end of the string
     - parameter length: Negative length leaves that many characters off the end of the string. Omit to return everything through the end of the string.
     */
    func substr(offset: Int, length: Int = 0) -> String {
        let start = offset < 0 ? endIndex.advancedBy(offset, limit: startIndex) : startIndex.advancedBy(offset, limit: endIndex)
        let end = length > 0 ? start.advancedBy(length, limit: endIndex) : endIndex.advancedBy(length, limit: startIndex)
        return substringWithRange(start ..< end)
    }
}