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
Fixes to rewrtie after testing
Basic validator code
Bug fixes
Add badic validation logic
Validation
Committing progress
We progress
Fix non-self-closing hr + br tags
Seemingly working validator
Add b,i,em,strong,br,hr tags to validator
Permit table>tr as well as table>tbody>tr
Make tag detection case insensitive