Allow bdo tag through wp_kses()

[iandunn]

The ​bdo tag provides a semantic-free method for switching text-direction within a single page. e.g.,

<p>This text will go left to right.</p>
<p><bdo dir="rtl">This text will go right to left.</bdo></p>

Support for it was silently added in r2896, and then removed without much justification in r2924.

I think there are valid use cases for the tag, and don't see any reason to prevent people from using it.

[iandunn]

After reading more, it seems like <div dir="rtl"> is the better way to handle the uses cases I was thinking of, and it's already allowed. <bdo> is ​intended more for edge cases.

It should probably still be allowed, but is much less of an issue than I originally thought.

[wonderboymusic]

In 35141:

KSES: have you ever heard of the <bdo> HTML tag? Same. ​http://www.w3schools.com/tags/tag_bdo.asp

Adds unit test.

Props iandunn.
Fixes #34063.