Add dtr property to SerialPort

Sources/SwiftSerial.swift

@@ -217,6 +217,7 @@ public class SerialPort {
 
     var path: String
     var fileDescriptor: Int32?
+    private var dtrState = false
 
     public init(path: String) {
         self.path = path
@@ -365,6 +366,24 @@ public class SerialPort {
         }
         fileDescriptor = nil
     }
+    
+    public var dtr: Bool {
+        get{
+            return dtrState
+        }
+        set (value){
+            // TODO: Add error handling.
+            guard let fileDescriptor = fileDescriptor else {
+                // Need to open port first.
+                return
+            }
+            dtrState = value
+            var flags = TIOCM_DTR
+            if(ioctl(fileDescriptor, dtrState ? TIOCMBIS : TIOCMBIC, &flags) != 0){
+                
+            }
+        }
+    }
 }
 
 // MARK: Receiving