Enforce new tab (`target="_blank"`) via `UILinkParams`

[buschtoens]

Relates to #6 (support for Cmd / Ctrl click).

Ideas:

• (link "foo" newTab=true)
• (link "foo" target="_blank"), support for generic targets?
• It would be nice, if target could be applied to the <a>.
  • A modifier for jointly applying all attributes and click listener?
  • target property on Link / UILink?
  • Should the click handler respect the target attribute of the <a> tag?
• Should we allow the "call site" to opt-out?
  • We could use transitionTo / replaceWith as an explicit opt-out.
  • Add a more generic navigate.
  • Relation to External Links #290 (external links)?