Add PDF option to build command

[raqbit]

The README.md currently mentions the following:

Using assets, you may specify additional assets to copy into the build dist directory in case they are needed for the final render to PDF.

The original use-case for docmaker is to generate HTML files from Markdown which can then be used with https://weasyprint.org/ to generate nice looking PDFs. Currently the PDF conversion part is manual, which means you'll have to run weasyprint build/index.pdf build/out.pdf to get your PDF output.

It would be nice if we could add a flag for automatically calling weasyprint for the user, this way building your document is only 1 command in total and the user does not have to use (bash/batch) scripting to get an easy build the document command.