Not only does this remove some unsafe code from clap itself, `atty` does the
right thing on Windows too. This isn't relevant now since we don't currently
support colorized output on Windows, but will come in handy if/when we
implement that feature (#836).
Color are now only used when outputting to a termainal/TTY. There are three new settings as well
which can be used to control color output, they are:
* `AppSettings::ColorAuto`: The default, and will only output color when outputting to a terminal or TTY
* `AppSettings::ColorAlways`: Outputs color no matter where the output is going
* `AppSettings::ColorNever`: Never colors output
This now allows one to use things like command line options, or environmental variables to turn
colored output on/off.
Closes#512
