Add support for credentials and site via Git configuration

This patch adds support for supplying credentials and site via `git config`.

Example:

```bash
$ git config vae.username someuser
$ git config vae.password secret
$ git config vae.site mysite
```

With this in place one can simply run `vae` to start a local development environment using the  credentials and site from the Git configuration.

To make things even simpler and more secure, the credentials can also be stored in the global Git configuration (which is usually only readable by the current user):

```bash
$ git config --global vae.username someuser
$ git config --global vae.password secret
```

This way credentials are shared among all sites, this can naturally be overridden by a local configuration option(s).