Implemment lookaheadreader with lookahead and commit api

The LookAheadReader api works similar to read but it has 2 methods,
lookahead: it will read 1 byte (with internal ftell) without modifying
the file's own ftell() and commit() which saves the current file ftell

LookAheadReader can wrap both buffers and files

src/header/stdio/mod.rs

@@ -39,6 +39,7 @@ mod getdelim;
 
 mod ext;
 mod helpers;
+mod lookaheadreader;
 mod printf;
 mod scanf;