feat: getContainer return the container element

Update README with `getContainer` specs and fixes.
fix(filter): propagate `filtered#` changes to generated selects too

Clearing a filter did not propagate the filter changes back to the generated selects,
so when trying to move an option (that didn't previously match the filter) right after
clearing the filter, caused nothing at all.
This was due to the move/remove/moveAll/removeAll functions relying on the
`filtered#` data attribute on the generated selects, not the original one.
Merge branch 'hotfix/filter-select'