Columns beginning with an underscore do not facet correctly

Datasette treats columns that start with an underscore as querystring parameters it should ignore!

<img width="914" alt="bchydro__item_versions__99_918_rows" src="https://user-images.githubusercontent.com/9599/141664864-119990d5-42b2-47f7-91bd-09a74a288af9.png">

Discovered in https://github.com/simonw/git-history/issues/14#issuecomment-968192464