Support for filters

[hugoroy]

I have tried to use a filter using the following YAML metadata in a page:

latex_extra_options:                                                                                                                                           
  - -\-filter=~/Modèles/pandoc/clean.sh                                                                                                                        
  - -\-filter=pandoc-latex-environment                                                                                                                         
  - -\-filter=~/Modèles/pandoc/latex.sh  

It didn't seem to work. Is it supposed to?

By the way, I opened issue #2 and finally got time to test. It works great! Really awesome work, so thank you! I am definitely switching :-)