added 'in' textfilter operator

This patch adds support for an 'in' operator with FILTER_TEXTSEARCH. It accepts a comma-separated list of values to match against, with support for '%' wildcards. 

In order to support wildcards in a portable way, the filter doesn't actually use `in()` in the SQL it produces, but instead creates a statement like 'AND (var like val1 OR var like val2...).