Fix not to overwrite something unexpectedly

[tagomoris]

Originally, some plugin helpers (e.g., storage, formatter, parser, ...) defines config_section for sets of parameters by included hook. It calls config_section with plugin class as receivers.
But, if plugin class calls config_section to override default value, that overwrites the section definition defined by plugin helpers.
It makes many unexpected behaviors (including breaking instance variables).

This change is to prevent overwriting section definitions by plugins. By inserting a mixin module, config_section definition in plugin classes overrides definitions in plugin helpers, instead of overwriting it.

[tagomoris]

@repeatedly Could you review this?

[repeatedly]

okay. I will review it later.

[repeatedly]

LGTM 👍