Added a new block called "admin_title" that allows for greater customization of the main header title

This tweak came about when I had the need to put an image next to the title displayed on the main header.  I can make it work with the current version doing this...

```python
GRAPPELLI_ADMIN_TITLE = "<img src='logo.png'> Admin Title"
```

But I'd rather be able to further customize the html in the header...also, the solution above seems a little kludgy.

To me, it makes more sense to be able to put this in the admin base_site.html

```html
{% block admin_title %}
<img src='logo.png'> Admin Title
{% endblock %}
```

PS: This is my first time contributing to a project on git.  If I did any of this incorrectly, please forgive me :)