Check whether stderr is a tty if writing to stderr

While writing up #48 I found a bit of an odd behaviour in tunnelto: although it apparently writes its output to `stderr`, it doesn't check whether `stderr` is a tty, instead it checks stdout:

$ tunnelto 2>test.txt

will write a bunch of ANSI escape codes to `test.txt`, but

$ tunnelto > /dev/null 2>test.txt

will not.

So when writing a script around tunnelto, despite only reading from stderr one has to redirect `stdout` in order to avoid an output full of ANSI.