Add :email and :password options to `aptible login`

attn: @bwest87

This adds the ability to log in noninteractively (e.g., on CI) by specifying `--email` and `--password` on the command line.

For security, I'd recommend storing the password in a file, and running this as:

```
aptible login --email test@example.com --password $(cat password.txt)
```