Add a way to do left bound word search to chosen for arbitrary word bounardies

I wanted a way to search on a dropdown that had separators dev_db.time_on_site_logs, and wanted left bound search on period / space / underscore. This supports that and a bit more -- you can use the particular separator to only left-bound your search on that separator so _logs vs .logs.

Not super thrilled about the name / docs, but happy to work on that if you like the idea.