Use the right name

src/cli.rs

 
 pub fn new_gist(matches: &ArgMatches) -> io::Result<()> {
     let title = matches.value_of("title").unwrap();
-    let input = matches.value_of("input");
+    let input = matches.value_of("file");
     let filetype = matches.value_of("filetype");
 
     let mut rdr: Box<io::Read> = match input {