There are no simple way to initialize LineDecoder in Ruby. However, a lot of parser plugin (especially simple plugins) are based on lines. Plugins should not have these lines: https://github.com/treasure-data/embulk-parser-query_string/blob/261cec3a69d9ecf840fb560e4f36c6d0c53138d3/lib/embulk/parser/query_string.rb#L13
A discussion point is the design of bridge or mixin. This design can be reused when we want to have many other utilities for each domains.