Fix the output of the JS `markdown` executable.

Previously, because of `console.log` semantics, if you ran:

```
make test PROG=js/markdown
```

You'd get a couple of errors caused by `console.log` calls putting `\n`
line breaks where they shouldn't. This fixes that by using
`process.stdout.write` instead of `console.log`.

This isn't something really important, but it's nice for all the
provided executables to pass the test suite.