Add Pulse DTR command

MCUs like the ESP32 can be reset if the serial port DTR line is
pulsed for a short time.  You could just type CTRL-t d CTRL-t d
but that's a little awkward since you have to lift your finger
off the CTRL key to type the Ds.  Now you can just type CTRL-T D.

* Added new command "D" to pulse the DTR line.  I.E.  Toggle its
  state twice with a configurable duration between toggles.

* Added new config/command line option "--dtr-pulse-duration"
  to set the duration between the DTR state toggles.  The default
  is 100ms.

src/bash-completion/tio.in

@@ -16,6 +16,7 @@ _tio()
           -s --stopbits \
           -p --parity \
           -o --output-delay \
+             --dtr-pulse-duration \
           -n --no-autoconnect \
           -e --local-echo \
           -l --log \
@@ -58,6 +59,10 @@ _tio()
             COMPREPLY=( $(compgen -W "0 1 10 100" -- ${cur}) )
             return 0
             ;;
+        --dtr-pulse-duration)
+            COMPREPLY=( $(compgen -W "10 50 100 500" -- ${cur}) )
+            return 0
+            ;;
         -n | --no-autoconnect)
             COMPREPLY=( $(compgen -W "${opts}" -- ${cur}) )
             return 0