Allow using a different partial than the default `<association>_fields`

[viamin]

Adds a :partial_name option which allows using a differently named partial.

Usage:

link_to_add_association "Add a new task", f, :tasks, partial_name: "task_dropdown"

which will use the _task_dropdown.html.erb partial instead of the default _task_fields.html.erb

[hungle00]

That's a good change. Thank you!