Add an unstable option to print all unversioned files

This allows sharing those files between different doc invocations
without having to know their names ahead of time.

src/librustdoc/html/render/mod.rs

@@ -33,6 +33,7 @@ mod print_item;
 mod write_shared;
 
 crate use context::*;
+crate use write_shared::FILES_UNVERSIONED;
 
 use std::cell::{Cell, RefCell};
 use std::collections::VecDeque;