Add support for Slim SEO

[rilwis]

Add support for the Slim SEO plugin, which will exclude tags with fewer than the minimum number of posts from the Slim SEO sitemap.

Slim SEO uses a similar filter as the core sitemap, thus the code is similar.

I've tested and it works as intended.

[rilwis]

Hi @jdevalk @aristath , can you please take a look?

[aristath]

Hey there @rilwis!
Thank you for this PR, and my apologies that it took us so long to get to it... It was submitted during the WCEU weekend and we missed it 😅

• Code looks good
• Reviewed the code in simple-seo, and saw that the exclude arg gets passed-on to get_terms(), so this looks OK
• Automated tests pass

cc @jdevalk

[jdevalk]

Code looks good, tested too, works as intended.

[rilwis]

Thank you!