added shell_command_is_shell option into terminal config , saving one sh...

Hi Daniel.

I Would like to contribute to your great project.

As you know we are working with gateone in embedded mode in the ITSAT project and we need to exec a external command (  in  our case a php  console script  /usr/bin/php5 -f <our_console_scrip> )

if we put this config

           "commands": {
     
                "ITSAT-SHELL": {
                    "command": " /usr/bin/php5 -f /opt/itsat/usr/bin/rcpsh",
                    "description": "Connect to hosts via ITSAT SHELL."
                }
            },

On each opened terminal we had this process tree

<pre>
gateone,2175 /usr/local/bin/gateone --settings_dir=/opt/itsat/etc/gateone
  +-sh,3465 -c /usr/bin/php5 -f /opt/itsat/usr/bin/rcpsh; sleep .1
      +-/usr/bin/php5 -f /opt/itsat/usr/bin/rcpsh ,3466
</pre>


Now I can Configure GateOne Terminal 

           "commands": {
                "ITSAT-SHELL": {
                    "command": "/opt/itsat/usr/bin/rcpsh",
                    "description": "Connect to hosts via ITSAT SHELL."
                }
            },
            "shell_command": "/usr/bin/php5 -f",
            "shell_command_is_shell": false,

And the process tree is 

<pre>
 python,4302 /usr/local/bin/gateone --settings_dir=/opt/itsat/etc/gateone
  +-/usr/bin/php5 -f /opt/itsat/usr/bin/rcpsh ,4424
</pre>

This commit saves 1 process ( a shell ) by opened terminal when you want to exec another binary command that is not a shell.( php5 in my case) and is necesary to avoid the sleep .1 attached to the command. And also enables faster executions.