Add customizable page transitions

[dotargz]

• allow for a new entry into story.json to change the page transition from the default

[devxan]

https://swup.js.org/ 💞
Also maybe include the accessibility plugin 🤔
Here are the different transitions (called themes by swup)

Seems like you're planning to have a set global transition in metadata json object.
Is it possible to override transitions in the page object for setting entrance/exit transitions?