Opens filename for output in text mode. Raises an error if it cannot.
If opened OK returns a handle to the opened file, and makes it the default output file.
If called with no argument, returns the handle to the default output file.
If called with a file handle, sets the default output file to that handle.
f = io.output ("test.txt") --> handle to new file
f:write ("some data here") -- write to it
f:close () -- close that file now