Filter values that only differ in case, like `Mint` and `mint`, came through as separate values. The new `--lowercase-filters` option (`lowercase_filters`, `PAGEFIND_LOWERCASE_FILTERS`, and `lowercaseFilters` / `lowercase_filters` in the Node and Python wrappers) lowercases filter values when indexing and merges values on a page that become equal. Filter names are left as they are. Off by default. Fixes #737