Set fonts based on distribution

Not all default fonts used by seq2vid are available on all distributions, and even if they are, font variants (bold , italic, etc.) have different names.

Check the GNU/Linux distribution seq2vid is running on (using lsb_release -a probably), and change the default fonts used in plain.tmpl accordingly.

Or maybe better, directly test if fonts are available, and set them if they are. (How to check for font availability?)