Enable chaining for attribute accessor methods

```js
var tr = trumpet();
var href = 'http://example.com';

tr.select('a')
  .setAttribute('href', href)
  .createWriteStream()
  .end(href);

tr.pipe(process.stdout);
tr.end('<a></a>');
```
output:
```
<a href="http://example.com">http://example.com</a>
```
Let me know if this feature worth testing.