First of all, thank you Takayuki for this extremely useful plugin!
I have noticed one detail that could perhaps be improved: the plugin is adding some custom fields to WordPress, that are showing up in the Custom Fields dropdown menu. They are visible when I want do add my own custom fields to a post, for instance. The fields that I noticed are:
form. All those fields, it seems, are created by Contact Form 7.
I believe that it would be better if those custom fields would start with "_" (underscore), or maybe even better "_wpcf7_", so that they would be hidden from the user.
That's what is recommended in the WP documentation:
I hope this can be added as a fix in a future version of the plugin.