Support modifiers

E.g., `[key=value i]` for case insensitive values, `[KeY=value s]` for case sensitive attributes.

per https://twitter.com/estellevw/status/1132546213226602498?s=21

The issue names no files or tests. Start by locating the selector parser's attribute-selector handling and existing attribute-selector coverage, then compare the syntax described in the issue with the parser's current behavior. Done means the case-insensitive and case-sensitive modifier examples are accepted and covered by tests.

Written by the indexing model from the issue text.