Adding var IsInit to the API to know if termbox has been init or not

Hello,

I'm creating a little library over termbox-go, and to be able to properly close termbox, I need to know if it has not been already closed somewhere else. (Because, with more than one Close(), the prompt is not given back withtout pressing ctrl+c (on linux) when the program exits)

So, I just added a variable to know the state of termbox.

Hope you'll accept it,
Regards,
Peekmo