feat: attempt to autodetect need for -N switch to nc

[wincent]

Based on discussion here:

#2

[wincent]

Can you test this out and see if it works for you @xiruizhao?

I'm not a fan of the complexity here, but we have a choice:

1. Do nothing (ie. leave code as it was before; maybe try to improve the docs).
2. Do what this PR does (ie. "sniff" output of nc -h to try and figure out whether to pass the -N switch); I don't have a Ubuntu-ish machine to test this on, so I don't even know if it works.
3. Maintain a list of operating systems and check against that with uname; not a fan of that approach because people can install different tools irrespective of their OS, and the check could become quite unwieldy (for example, checking for "Linux" won't cut it because some distros — eg. Amazon Linux, which I was able to test — don't require or support -N).

[xiruizhao]

Works on Ubuntu. I think this PR is the best solution. You can also update the README of clipper to warn people (so that they can take care of their .bashrc).

[wincent]

Thanks for testing it @xiruizhao. Will give it a shot (merged).

[wincent]

Doc updates here: wincent/clipper@451bac8