Colorize status code output

[hako]

This PR colorizes status codes outputted by riplink, makes it way better to identify dead links.

• Green - 200
• Red - 400-500

[coveralls]

Coverage remained the same at 85.859% when pulling 301bf1e on hako:colorize into ca3b3db on mschwager:master.

[mschwager]

Hey @hako, thanks for opening this PR!

If you're output is being overwhelmed by HTTP 200's can't you simply drop the -verbose flag and have riplink only print errors? I'm hesitant to accept this because I'm not familiar with its portability. Will this work with all, or most, Linux and OSX shells? What about Windows?

I appreciate you opening this - having colored output is often much easier on the eyes than a typical dark and dreary shell :) but I'd also like to keep riplink as portable as possible!

[hako]

@mschwager The -colorize flag respects this use case, I guess I didn't think of it that way. Regarding portability, I can add support for colorisation for other OS's. Perhaps using a portable terminal color library would be the best way to solve this. What do you think?