More ergonomic support for case insensitive keywords

[dangoodman]

It would be great to have a switch to enable a case-insensitive matching of the keywords.

For now, one can use regexes, e.g., "[Tt][Aa][Bb][Ll][Ee]". While it works, it hurts readability a lot if all of the keywords are defined this way.

A new filters.toml key, say "case-insensitive": true/false, could help with that.

Another option is to support regex flags, e.g., "/table/i".