Allow setting CDN, API URL and HTTP proxy settings in Hex config

This pull request allows users to set the following configuration settings from the config file:

- API URL
- CDN URL
- HTTP proxy
- HTTPS proxy

>Note: Sadly, setting the home directory from the config file is impossible.

This allows users to do the following:
```shell
$ mix hex.config api_url http://hex.internal.corporation.com
$ mix hex.config cdn_url http://cdn.hex.internal.corporation.com
```

:christmas_tree: :fireworks: Merry Christmas and a Happy New Year!
We should trim the trailing slash instead of documenting that it isn't allowed.