[Feature Request] Add options to configure flexsearch

[echojoshchen]

Flexsearch allows configuring the tokenizer, split pattern, and encoding. Adding support for these options will allow the plugin to be configured.

Example:

module.exports = {
    plugins: [
        ['fulltext-search', {
            encode: 'icase',
            tokenize: 'full',
            split: /\s+/,
        }],
    ],
};