Various improvements

Changes to the output

• 12-point fonts are actually 12 points now (they were larger before) (8cbb1be)
• Any image in a paragraph by itself, with a caption, will be rendered as a centered figure with that caption (#22)
• Code blocks now have light-gray backgrounds so they stand out more (#3)
• Accept GitHub-flavored Markdown and underlines. Now *italics* and _underline_, instead of _italics_.

Development changes

• Can now generate Debian packages (#12)
• Minimum valac now 0.48
  • Use valac from the Vala Next PPA on Travis