* Add additional hex mode formats

 - Support traditional hex output format such as:

  00000000  74 65 73 74 20 74 65 73  74 20 74 65 73 74 20 74  |test test test t|
  00000010  65 73 74 64 66 0a 61 0a  66 61 0a 66 0a 61 73 66  |estdf.a.fa.f.asf|
  00000020  64 61 64 73 66 61 73 66  64 61 73 64 66 61 64 73  |dadsfasfdasdfads|
  00000030  66 0a 61 73 64 66 61 64  73 66 61 73 64 66 61 73  |f.asdfadsfasdfas|
  00000040  64 66 0a 61 73 64 66 61  64 73 66 61 73 64 66 61  |df.asdfadsfasdfa|
  00000050  73 64 66 66 64 61 73 64  66 0a 0a 31 32 33 31 0a  |sdffdasdf..1231.|
  00000060  65 32 31 64 73 77 65 64  0a 0a                    |e21dswed..|
  0000006a

  With configurable width output (4, 8, 16 bytes etc.). E.g. config file:

   hex-mode-width = 16

  With and without timestamp support for each broken up line.

 * Add support for activity based time stamping in both normal and hex-mode.

   Will print a new timestamp if there is no input activity within the time
   defined in the configuration file, e.g.:

    timestamp-timeout = 200

   Time is in ms.

* Advanced line mode

  Current line mode only support backspace editing. Would be nice with arrow
  key navigation left/right and insert/overwrite support. Also history browsing
  pressing up/down.

 * Support for running external command

   Add key command e.g. 'ctrl-t r' which prompts user to run external command.
   The command will be run in a process which stdin/stdout is redirected to the
   serial port.

   This is the first step towards maybe also adding automatic support for
   x/y/zmodem data transfer protocols by calling external programs such as
   rb/sb, rx/sx, rz/sz, etc.

 * Allow tio to connect to socket

   After some more consideration I think it makes sense to support connecting to a
   socket as that will make tio be able to both serve a serial port via a socket
   and connect to it - it will be an end to end solution. In short we will be able
   to do the following:

   Host serial port on socket (existing feature):
    $ tio --socket unix:/tmp/tio-socket-0 /dev/ttyUSB0

   Connect to same socket (new feature):
    $ tio unix:/tmp/tio-socket-0

   Besides a bit of refactoring the following required changes spring to mind:

    * Socket mode and type of socket should be activated via device name prefix. For example:
      * UNIX socket: tio unix:<filename>
      * TCPv4 socket: tio inet:<ip>:<port>
      * TCPv6 socket: tio inet6:<ip>:<port>
    * If no port number defined default to 3333
    * Mapping flags INLCR, IGNCR, ICRNL needs implementation for socket mode
    * Error messages should just say "device" instead of "tty device" etc.
    * Remove other tty'isms (tty_write() should be device_write() etc.)
    * In session key commands that do not work in socket mode should either not be listed or print an error messages if used.
    * All non-tty features should continue work (auto-connect etc.)
    * Shell completion script update
    * Man page update


 * Split I/O feature

   Allow to split input and output so that it is possible to manage these
   independently.

   The general idea is to redirect the output stream on the socket port number
   specified but then redirect the input stream on the same port number + 1.

   Example:

    $ tio /dev/ttyUSB0 --socket inet:4444,split-io

   Will result in output stream being hosted on port 4444 and input stream
   hosted on port 4445.

   For file sockets something similar can be arranged:

    $ tio /dev/ttyUSB0 --socket unix:/tmp/tio-socket-0,split-io

   Will result in output stream being hosted via /tmp/tio-socket-0 and input
   stream hosted via /tmp/tio-socket-0_input

 * Websocket support

   Extend the socket feature to redirect serial I/O to websocket on e.g. port
   1234 like so:

    $ tio --socket ws:1234

   Use libwesockets to implement feature.