Specify rules in external YAML file

This is https://github.com/cereda/arara/issues/97

It would be nice to specify the build rules not in the .tex file but in an external .yml. For example:

fileA.tex:
   - pdflatex
   - bibtex
   - pdflatex:
      option
   - pdflatex
fileB.tex:
   - lualatex
   - biber
   - lualatex
   - lualatex