Adding direct PDF creation of beamer slideshows to Markdown-build

The PDF build in the current build-settings forces you to use only one output type, standard latex by default, but I needed to sometimes build beamer slide pdfs too so I've added that build type. Since you can just specify "latex" or "beamer" as target formats with a .pdf extension this means that the explicit treatment using the pdf_flag is actually not necessary, so I've changed the format of the old 'PDF' build command too.

If you're fine with these changes I can also add them to the two other build-settings (and potentially chuck out the self.makePDF stuff in the BuildCommand)

Many thanks for the package, it's awesome!