Add streaming / piping support.

This pull request adds streaming (node) / piping (cli) support. See the following usage examples:

Streaming:

```javascript
var browserify = require('browserify');
var unpathify = require('unpathify');
...
var b = browserify();
b.add('./browser/main.js');
b.bundle().pipe(unpathify()).pipe(process.stdout);
```

Piping:

```sh
$ browserify in.js | unpathify | uglifyjs -cm > out.js
```

I tried to do this in a way which matched the existing coding style and had minimal impact on the code as a whole.