Export to PDF

[timhoffm]

Add a method to Template:

def save_pdf(self, filename):
~~

Since `python-pptx` does not provide this capability. We need to rely on external programs (LibreOffice and/or PowerPoint).

LibreOffice can be used on the command line with

libreoffice --headless --convert-to pdf --outdir . filename.pptx

Tasks:
1. Check if LibreOffice is available on the system, if not fail
2. Create a temporary directory and save the tempate there as ´pptx`.
3. Run the above command in the temporary directory and target the output dir the user wants.
4. Cleanup

Optional:
Extend this to work with PowerPoint as well. Way of interaction with PowerPoint to be defined. I don't know if it can be controlled via command line.

[fahro]

#18