The URL can be set by either:
1. Setting the `baseUrl` property in `config/default.json`,
`config/local.json`, or `config/${NODE_ENV}.json` file.
2. Setting the `PEERCALLS_BASE_URL` environment variable.
For example:
```
$ PEERCALLS_BASE_URL=/test1/test2 npm start
> peer-calls@2.0.3 start /peer-calls
> node src/index.js
peercalls WebSocket URL: /test1/test2/ws +0ms
peercalls Listening on: 3000 +76ms
```
In this case, opening `http://localhost:3000/test1/test2` would open the
homepage.
Fix#42
