Beamer/PDF: add attachment automagically

[cagix]

when generating beamer slides or pdf handouts these artefacts should be linked within the document (gfm/docsify version). for pdf output this has been done manually, but it would be nice to have some kind of tooling support here instead.

1. add yaml variable "slides" and/or "handout"
2. generate additional makefile targets/variables (make.deps)
3. makefile targets beamer and pdf: do generate artefacts only for content in new variables
4. add attachment links in lua-filter. needs base-url (repo), branch, relative path to document

see #24