New option to select only one backend at a time.

Hi Sven,

Following the discussion we had about **backend_chooser** option that is used only in the config plugin, I did a hack that add a new option called **backend_single**. If you enable it, you will be able to select only one backend at a time. This does the trick but implementation may be bad, I fully understand that you refuse this pull request ;-)

* option name: backend_single (0 / 1).
* support config file added.
* there is a doc ! ;-)
* default value is disabled to not change default Thruk behavior.