Add parsedown library
Markdown pages support
Convert some pages to markdown in-place

They don't get rendered in this commit, renaming them in the next commit
to keep diffs visible here.
Rename markdown pages to .md
Remove whitelist of valid pages, use .php extension for php pages
Rename remaining pages to .php

Twitter page removed - it was just an include for twitter.php, which
isn't in the source tree for some reason.