Modify mockbin script to use env vars or custom npm config

[marcel-dias]

What ?

I'm proposing a modification in the ./bin/mockbin script to read the startup params from Environment Variables or from the npm config properties. Now it is correct reading the customized values.

Example:
If you set the port to 8001 with npm config set mockbin:port 8001 the server will listen the 8001 instead of 8080

Why ?

I was trying to use the docker image, but mockbin was not able to connect to redis instance. This happens because the bin/mockbin reads the config from pkg.config.* and this properties always have the default values from package.json.
So I modified it to read from process.env.npm_package_config_redis then the custom config npm config set mockbin:redis redis://redis:6379 will work properly.

Probably fix #48

[ahmadnassri]

good job, thanks!

will review and release after I fix the travis build ..