Add `--safe-output` mode.

In this mode, the program will test whether stdout referes
to terminal before adding ansi-sequences for highlighting.

This protect users from the getting ansi-sequences in the
redirected output such as:

    $ em author RED setup.py > test.txt

and breaks complex things like:

    $ tail -f /path/to/log | em ERROR red | em WARNING yellow

since the first em's instance will be refers to the pipe, not tty.