Allow one-sided ranges

[taylor-steve]

Looking to fix #235, if that original behavior is still desired.

The ability to specify only the start or end date was lost in https://github.com/projectblacklight/blacklight_range_limit/pull/179, which creates a nice system using ranges.

This PR looks to restore that old behavior but depends on making a small change to Blacklight to handle beginless/endless ranges (e.g., nil..2002). Something like this: https://github.com/projectblacklight/blacklight/pull/3213