add explicit content types to VFS

this allows us to serve a precise Content-Type header for all our pages, rendering treehouse usable in browsers that require one
(like terminal browsers---w3m, lynx)

crates/treehouse/tests/it/vfs/file.rs

@@ -1,7 +1,7 @@
 use treehouse::vfs::{entries, query, BufferedFile, Content, VPath};
 
 fn vfs() -> BufferedFile {
-    BufferedFile::new(b"hewwo :3".to_vec())
+    BufferedFile::new(Content::new("text/plain", b"hewwo :3".to_vec()))
 }
 
 #[test]