relevanssi_allow_one_letter_highlights

…that let you disable one-letter searches, if you need them (1.15.3 and 3.5.11 disabled the feature). New filter: relevanssi_allow_one_letter_highlights lets you allow one-letter highlights. Just make the filter function return true. New filter: relevanssi_block_one_letter_searches by default blocks one-letter searches. If you want to enable them, add a filter function that…