Feature: pluggable transforms

[sudodoki]

Would leave this here for discussion & as possible future plan.
Be able to add transforms in semi-private api. They should be invoked with

1. markup
2. each slide markup
3. each slide html
4. resulting html
   And returned values should take place of inital values (modified markup/html). Need to be sure to account for possibility to return multiple slides from one or none.