fix(windows): Add option to disable RTS (#1277)

* Add option to disable RTS on Windows
* Add support for XON / XOFF on Windows.  This is probably not feature complete in terms of missing the XonChar, XoffChar, XonLim fields of DCB, but I am not as familiar with this aspect of serial comms.

BREAKING CHANGE:
We previously hard coded to have RTS On for windows at all times.

src/serialport_win.cpp

@@ -89,11 +89,28 @@ void EIO_Open(uv_work_t* req) {
   dcb.Parity = NOPARITY;
   dcb.ByteSize = 8;
   dcb.StopBits = ONESTOPBIT;
-  dcb.fInX = FALSE;
-  dcb.fOutX = FALSE;
+
+
   dcb.fOutxDsrFlow = FALSE;
   dcb.fOutxCtsFlow = FALSE;
-  dcb.fRtsControl = RTS_CONTROL_ENABLE;
+
+  if (data->xon) {
+    dcb.fOutX = TRUE;
+  } else {
+    dcb.fOutX = FALSE;
+  }
+
+  if (data->xoff) {
+    dcb.fInX = TRUE;
+  } else {
+    dcb.fInX = FALSE;
+  }
+
+  if (data->rtscts) {
+    dcb.fRtsControl = RTS_CONTROL_ENABLE;
+  } else {
+    dcb.fRtsControl = RTS_CONTROL_DISABLE;
+  }
 
   dcb.fBinary = true;
   dcb.BaudRate = data->baudRate;