Added some String extensions.

Sources/Extensions.swift

@@ -46,3 +46,19 @@ extension Int {
     }
 
 }
+
+extension String {
+
+    subscript (index: Int) -> Character {
+        let charIndex = self.index(self.startIndex, offsetBy: index)
+        return self[charIndex]
+    }
+
+    subscript (range: Range<Int>) -> String {
+        let startIndex = self.index(self.startIndex, offsetBy: range.lowerBound)
+        let endIndex = self.index(startIndex, offsetBy: range.count)
+
+        return self[startIndex..<endIndex]
+    }
+
+}