[seo-check]: allow configuration of correct meta description length

[lukaskawerau]

The current limit is 120 characters, and if I want to please seo-check I stay under it – but that then often results in complaints from ahref, which recommends 110-160 characters.

I'd love to be able to both please seo-check and Ahrefs.

[nickreese]

@lukeed is working on a core rewrite of the rules. We'll look at making this more customizable when that is done.