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!
No colons in headers and I think using the word default is wrong here. What do you think about this?

```
 ## Config keys
  - `username` - Hex username
  - `key` - Hex API key
  - `api_url` - Hex API base URL (without trailing slash)
  - `cdn_url` - Hex CDN base URL (without trailing slash)
  - `http_proxy` - HTTP proxy server
  - `https_proxy` - HTTPS proxy server