added option on custom permalink structure management (%category%/%postname%)

[enapupe]

Added a new option on wp-admin/options-permalink.php

• Post category and name (sample-category/sample-post/)

I believe there is a reason to still not have this option. Maybe the fact that %category% works not clearly when the post has more than one category. Anyway, I have seen many (most of them) blogs using this structure, so, why not make it easy to set it up?
This is my first patch submission, have mercy on me.

[SergeyBiryukov]

Related: #20172

[dd32]

For now, this isn't something the core team wants to add. We may reconsider it later on when the code and querying related to that permastruct is cleaned up and more reliable. Closing as maybelater for now.