Including a title graphic on a beamer template might require some
options, like e.g. `height=1.5cm` to ensure some image will fit at its
place in a given template.

I suggest to add an option for this use case.

Without it, my current workaround is:
```yaml
header-includes:
- \titlegraphic{\includegraphics[height=1.5cm]{media/cdl23.pdf}}
```