Support feeding whole document atonce

``` c
pj_feed(&parser, doc, doc_len);
pj_feed_end(&parser);
pj_poll(&parser, tokens, tokens_len);
```

Ensure this code works as most developers would expect. I.e. `tokens` will be filled with all tokens according to `doc` up to the final `PJ_END` (or `PJ_ERR` if applicable).
