Add support for PHP files being parsed at build time

Files with the extension %.html.php will be executed during the build step
and be included in the output as just %.html
Convert history index to .html.php
Support rewriting betwen .html and .php
Make various source files static .htmls
Fix a Futurama reference
Fix page links on Picocon 12 History page
Make Tube line colours clearer on Picocon page
Remove index.php/.html from urls