Sets an unique identifier for this element.
This identifier is used in target derivation
to create a CSS selector that matches this element precisely.
If the element already has other attributes that make a good identifier,
like a good [id] or [class] attribute, it is not necessary to also set [up-id].
Example
Take this element:
<a href="/">Homepage</a>
Unpoly cannot generate a good CSS selector for this element:
up.fragment.toTarget(element)
// throws error: up.CannotTarget
We can improve this by assigning an [up-id]:
<a href="/" up-id="link-to-home">Open user 4</a>
The attribute value is used to create a better selector:
up.fragment.toTarget(element)
// returns '[up-id="link-to-home"]'
