Add a library for a specific block

The current functionality loads any active library on every page, but a common use case is JS inside a block referencing a library where the block only appears on one or a few pages. In order to conditionally load JS, one would have to use PHP in the block to call drupal_add_js(), or hard-code the link to the JS in the markup (losing the advantage of e.g. loading the JS in the page footer).

The suggested feature is to allow within each block config page the selection of one or more of the libraries that is otherwise inactive (i.e. not loaded on every page).

As far as implementation, this module could use hook_block_view_alter() to add the JS for each configured block.

Note there would probably need to be some method for resolving conflicting config (one block nees the libread in the HEAD, another in the footer), or perhaps we could enforce some simplifying assumptions such as putting all block-level JS in the footer?