ENH: Function to Convert PPTX to PDF

[muschellij2]

If LibreOffice installed, you can convert a PowerPoint to a PDF. I don't think any other functionality exists in R (not in pandoc or pdftools). Figured it'd be a nice addition since you had all the lo_* functionality in there.

Added a test that works on my machine with LibreOffice (pretty old). Don't know if you want to add

before_install:
  - sudo apt-get update -y
  - sudo apt-get install libreoffice-core

to the .travis.yml to be able to test on CI.

[muschellij2]

Let me know if you need me to add anything or you want me to revise anything @hrbrmstr