Fix behavior of --include and --output flags

It was not intended that these would consume multiple arguments after a
single instance of the flag.

Author: dtolnay

gen/cmd/src/app.rs

@@ -136,6 +136,7 @@ into the generated C++ code as #include lines.
         .short("i")
         .takes_value(true)
         .multiple(true)
+        .number_of_values(1)
         .validator_os(validate_utf8)
         .help(HELP)
 }
@@ -150,6 +151,7 @@ not specified.
         .short("o")
         .takes_value(true)
         .multiple(true)
+        .number_of_values(1)
         .validator_os(validate_utf8)
         .help(HELP)
 }