Allow blog posts to specify an explicit URL slug

[berberman]

I have some posts with non-English titles, but I'd like their URLs to use short ASCII slugs. Currently the sulg is computed from the post title:

verso/src/verso-blog/VersoBlog/Basic.lean

Lines 222 to 231 in ffd58ff

	def defaultPostName (date : Date) (title : String) : String :=
	s!"{date.year}-{date.month}-{date.day}-{slugifyTitle title}"
	structure Config where
	destination : System.FilePath := "./_site"
	showDrafts : Bool := false
	postName : Date → String → String := defaultPostName
	remoteInfoConfigPath : Option System.FilePath := none
	verbose : Bool := false
	deriving Inhabited

While overriding postName can customize the generated name, it only gets the date and title, so the slug has to live somewhere else. And more importantly, Config now seems to be an internal thing:

verso/src/verso-blog/VersoBlog.lean

Lines 843 to 849 in ffd58ff

	def blogMain (theme : Theme) (site : Site) (linkTargets : Code.LinkTargets TraverseContext := {})
	(options : List String) (components : Components := by exact %registered_components)
	(header : String := Html.doctype) :
	IO UInt32 :=
	withLogger fun logger => do
	let cfg ← opts {} options
	let (site, xref) ← site.traverse cfg components |>.run logger

There's no way to pass it as argument, so I'm not sure how to modify postName.

I thought it would be nice to add an optional slug to Post.Meta, so this would get /blog/1980-01-01-test/ naturally:

#doc (Post) "测试" =>

%%%
authors := ["foo"]
date := { year := 1980, month := 1, day := 1 }
slug := "test"
%%%

But I'm not sure if there's already a good way to extend post metadata downstream (without defining a new blog genre).