Update terminal.py

Changed the if, elif, else logic for get_terminal_size(). When I am testing code in an IDE on Windows, the typical Windows code fails to work and get_terminal_size() returns None--which throws an exception later in the code. The quick fix is to modify the logic so that if we don't have a method for determining the terminal size OR if any of our known methods fail, we just assume a standard terminal. It might be better to add more error handling downstream of this method in the execution, but assuming a standard terminal definitely seems like a 'good enough' solution to me.
Merge pull request #129 from dbotwinick/patch-1

Update terminal.py