Support GIST indexes in `custom_indexes` code block

    custom_statements do
      statement :my_index_1 do
        up "create index my_index_1 on properties using gist (geography);"
        down "drop index my_index_1;"
      end

      statement :my_index_2 do
        up "create index my_index_2 on properties using gist (geography(geography));"
        down "drop index my_index_2;"
      end
    end