Allow config.json location to be configured by argument to app.js

Hi,

Following on from #137, this change allows the location (and name) of config.json to be configured via a command line argument to app.js. This allows I/O Docs to be run without any modifications within the repository.

The downside is that `npm start` can't be run with arguments (they don't get passed through to the specified script), until [issue 3494 in npm](https://github.com/isaacs/npm/issues/3494) is resolved. Instead, supervisor (or node) has to be run manually.

I know that's not ideal, but I think this is a big enough win to justify the pain (until the npm issue is resolved).

Thanks,
Rowan