Refactor hostname to be a runtime option

Add a `hostname` field under Server in your conf file, see the
updated nitter.conf in the repo for an example. The compile-time
option (-d:hostname) is no longer used.

src/routes/router_utils.nim

@@ -2,7 +2,7 @@ import ../utils, ../prefs
 export utils, prefs
 
 template cookiePrefs*(): untyped {.dirty.} =
-  getPrefs(request.cookies.getOrDefault("preferences"))
+  getPrefs(request.cookies.getOrDefault("preferences"), cfg.hostname)
 
 template getPath*(): untyped {.dirty.} =
   $(parseUri(request.path) ? filterParams(request.params))