Marks a target selector as optional.
If a :maybe selector is not found in the current page or the server response,
Unpoly will skip rendering the fragment instead of throwing an error.
When updating multiple fragments
you may combine required and optional selectors in a single target string.
Example
This link will update the fragments .content (required) and .details (optional):
<a href="/card/5" up-target=".content, .details:maybe">...</a>
Only the fragment .content is required to match. If .details is missing
in the current page or the server response, Unpoly will only update .content
without an error.
