Make the cmdrunner more interactive

[noxer]

Currently the cmdrunner executes the command once for each update interval which delays the update for other widgets and prevents users from using longer running commands (like watching a log or requesting data from very slow servers). This PR fixes the problem by running the command concurrently and updating the view each time the command outputs data.

It also adds two config parameters:

• tail which causes the widget to scroll to the end of new output (interesting for following log files)
• maxLines which limits the number of lines kept in the buffer (to prevent the widget from consuming unlimited memory)

Also the widget is now focusable to allow the user to scroll in the output.

[noxer]

I've tested the new runner with dmesg on Linux.

    dmesg:
      args: ["--color=always", "-w"]
      cmd: "dmesg"
      tail: true
      enabled: true
      maxLines: 100
      position:
        top: 0
        left: 1
        height: 4
        width: 2
      refreshInterval: 30
      type: cmdrunner

[senorprogrammer]

Looks good!