Add option --script-language to explicitly set javascript or coffeescript

For #11744 I want to write a script in coffeescript and give it a shebang line of `#!/usr/bin/env phantomjs`. Unfortunately there's no way to specify the language of the script, so it would have to end in `.coffee` to be interpreted as coffeescript.

This PR add `--script-language` to override `.coffee` file extension matching.

On some platforms it will be possible to use the shebang `#!/usr/bin/env phantomjs --script-language=coffeescript`.