How about implements Fluent::TextParser::TypeConverter as mix-in function?

It is very useful for type converting with in_tail plugin or using fluent-plugin-typecast. But for now, other plugin couldn't use this module as mix-in plugin.
I have create [fluent-mixin-type-converter](https://github.com/y-ken/fluent-mixin-type-converter) to act calling [Fluent::TextParser::TypeConverter](https://github.com/fluent/fluentd/blob/master/lib/fluent/parser.rb#L54) as mix-in plugin.

However, It is better to provide this function as build-in mix-in.
Would you please give me opinion about this issue?

Thank you.