Allow dynamic focus target

[daun]

Describe the problem

• Some visits don't need to reset focus to the main element, e.g. fragment visits like filter UIs
• Ideally, the custom focus target could be overwritten per visit
• The fragment plugin and users can then just set visit.focus = '#list' or similar

Describe the proposed solution

• New key in visit object: visit.focus — or maybe nested/grouped somewhere?
• New hook content:focus that can be replaced

How important is this feature to you?

• [ ] Nice to have
• [x] Would make my life a lot easier
• [ ] I cannot use swup without it

[hirasso]

Very nice idea!