Add config via environment variables

For now, am only using them to point to the desired API URL, but can be used for anything.

From the `README.md`:

Configuration values (for example the URL of the Tidepool Platform) are set via environment variables. If you need to add a config value, modify the `.config.js` file. If you need to read a config value inside the app, use `var config = require('./lib/config')`. To set config values (do this before building the app), you can use Shell scripts that export environment variables, for example:

```bash
$ source config/local.sh
$ npm start
```