Nested and arbitrary sorting and empty data pagination controls error

[ChrisHacker]

Fixes sorting to a) work with nested data paths, b) to sort on values other than the column-key value, and c) to avoid failing to render when the data subscription is empty because the default-pagination-controls component throws an error.

[ChrisHacker]

Let me know if you're willing to merge the request and I can update the docs to reflect the changes before you do.

[kishanov]

Looks good! If you'll update the docs I'll release 0.5.2 with this new feature.