Nested interpreters should have exit command by default

[jcubic]

I have an idea for a new feature for jQuery Terminal

Just notices that exit command was not defined when using terminal::push()

this is the code responsible for setting exit:

exit: !!(options && options.login || !options)

exit should always be defined for nested interpreters unless exit is explicitly set to false.

We can also add exit option to push().

[jcubic]

Maybe it will be good idea to keep top level exit only when login is used. But for nested interpreters it should be defined.

Maybe even remove this error message

canExitError: "You can't exit from main interpreter",