Fix Indexes not being UTF16 (Emoji bug)

[wes-nz]

📝 Description

Having emoji breaks the parsing. The Indexes used were not UTF16.
This is fixing the issue #28 / #29

👎 Existing Behaviour

let breakingString = " *hello* .  *hello*  cncnc cncnc  *hello*  hmm\n  *hello*  🦌🐎🦌🐎\n  *hello*  Screen Shot 2017-10-12 at 11.04.58 AM.png  *hello*  vVe2keakU8.gif  *hello*   *hello* "

var parser: Parser = {
    let parser = Parser()

    parser.add(pattern: MDBoldPattern()) { (string, attributes) -> MatchedResponse in
        return MatchedResponse(string: string, attributes: [NSAttributedStringKey.font: NSFont.boldSystemFont(ofSize: 20)])
    }
    
    parser.add(pattern: MDEmphasisPattern()) { (string, attributes) -> MatchedResponse in
        return MatchedResponse(string: string, attributes: [NSAttributedStringKey.font: NSFont.boldSystemFont(ofSize: 20)])
    }
        
    return parser
}()

👍 Expected Behaviour Now