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/types.nim

@@ -172,8 +172,9 @@ type
     address*: string
     port*: int
     useHttps*: bool
-    title*: string
     staticDir*: string
+    title*: string
+    hostname*: string
     cacheDir*: string
     profileCacheTime*: int