Add option to use color codes in non-tty contexts

This adds the `--color=<when>` option to the command to specify when the output should be colorized, where `<when>` is either `auto`, `always` or `never`.

Fixes #325.
This line is where colorize is getting passed into whichever reporter has been selected. A few lines above this you can verify `options.argv.color` is a valid value, check for `options.argv.nocolor` and `process.stdout.isTTY` so that by the time you get down to this line you just pass a single boolean value. At that point all of the code that was changed over in `src/reporters/default.coffee` can be removed.
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