Add --color option

[matthew-healy]

This flag has three possible values: auto, always and never. Its default is 'auto'.

With 'auto' selected, we allow the codespan_reporting and rustyline libs to determine whether to show colors based on the current terminal settings.

With 'always' selected, we force-enable colors in both error reporting and the repl.

With 'never' selected, we disable all colors in output.

[vkleen]

This look reasonable. I think newer versions of clap have a way of automatically deriving the command line parsing for Enums; that would also expose the possible values in the help message. But that would require updating to a new clap version and using the derive feature there instead of structopt, I gues.
Maybe we should do that at some point? For now, this PR looks good enough to me.

[matthew-healy]

@vkleen I agree that replacing structopt with clap is overall a good goal.

I think that we might need to rethink the architecture of the binary's main file, the Program struct and the repl function first though. Currently I think it'd be impossible to make that work without either leaking clap into the Nickel lib, or leaking rustyline and codespan_reporting's colour types into the binary. I guess the latter isn't the end of the world, but I feel like there's probably a cleaner separation we can achieve if we move things around a little bit.