Progressive Load option does not permit relative selectors

[developher-net]

Using mutliple instances of Progressive Loading on a page is problematic, because the progressive option expects a selector that will have unique results across the entire page:
"#uniqueid" or ".class .unique"

This is different than the prev/next options, as those both allow for a relative selector:
"> .nonuniqueclass"

The prev/next options allow for relative selectors precisely because there may be multiple instances of a carousel on a page with similar class names, especially if output by a CMS.

By comparsion, the unique selector approach for the Progressive option prevents a similar use-case.

To be consistent, and to make a difficult to use feature easier, the progressive option should allow for relative selectors.