Fixed Bug with shell and shellArgs being callbacks

[TWood67]

I found this feature request and attempted to use it: https://github.com/chjj/tty.js/issues/62

I received some errors upon using the example provided. There are pieces of logic in the config.js and tty.js that expect shell and shellArgs to be of type string. I have updated the logic to not perform string operations when the two properties are functions.