First sweep trying to make all references to kylewm customizable

There's some fluff left (e.g. twitter.js). Work in progress...
maybe `{% include "custom_body_bottom.html" ignore missing with context %}`?
Also have `custom_head_top.html`, `custom_head_bottom.html`, `custom_body_top.html`.
I'll put your piwik code as `custom_body_bottom_example.html` and `.gitignore` all `custom_*` files.
OK?