Switch to using nconf for configuration

See https://github.com/indexzero/nconf

`nconf` supports:
1. Command-line arguments
2. Environment variables
3. A file located at 'path/to/config.json'

Environment variables is important for automated deployment such as Docker ( see #19 , #18 ) and testing on Travis CI ( #10 ), etc. Currently only the `_config.json` file is supported. I would like to standardize the options and allow command-line, environment variables, along with the optional file support.