--charset doesn't work for stdin

The compiler always reads stdin as UTF-8, regardless of `--charset`.

```bash
$ echo "var foo = '⌘';" | enconv -L none -x UCS-4 > test.js
$ java -jar compiler.js --charset=UTF-32 test.js # good
var foo="#";
$ java -jar compiler.js --charset=UTF-32 < test.js # bad
stdin:1: ERROR - Parse error. Character '' (U+0000) is not a valid identifier start char
var foo = '#';
^

1 error(s), 0 warning(s)
```

(Note that UCS-4 and UTF-32 are synonyms.)