Add --proxy setting (using http-proxy)

[jantimon]

This pull request implements the same feautre like #133 .
However it uses the http-proxy module instead of the proxy-middleware module.

From #133 :

This pull request would add a new feature to http-server.
It is called proxy as it uses the proxy-middleware module to redirect unresolvable requests to a given url.

Checkout the pull request:

git clone https://github.com/jantimon/http-server.git --branch feature/node-http-proxy --single-branch --depth 1
cd http-server
npm install

Example usage

node bin/http-server -P http://blog.nodejitsu.com/

Open http://localhost:8080/img/ and it will resolve http-server/public/img just like before.

Next navigate to http://localhost:8080/nodejitsu-joins-godaddy/ - as it cannot find the folder nodejitsu-joins-godaddy/ locally it will redirect the requests to the proxy address and you will see the blog post:

My motivation behind this pull request is debugging live pages
Lets say you have a web page running on http://somepage.com/ and there is a bug in http://somepage.com/example.js. All you have to do is placing a patched or debug version of example.js in a folder and start up http-server with a proxy to http://somepage.com and open http://localhost:8080

The proxy feature works even better if you disable the directory listening -d false but this is not required.

[indexzero]

Looks good. if the Travis build is OK then I'll merge this in 👍

[jantimon]

Cool thanks