Exception on WP import looking for <pre> tag

Another quick one.  In `decode_wp_content()`:

```python
start = pre_part.index("<pre")
```

should be:

```python
start = pre_part.find("<pre")
```

because the next line checks for `start == -1`, but `index()` throws `ValueError` when the value can't be found, whereas `find()` returns -1.

I can send a PR for this tomorrow, but wanted to make sure I reported it tonight.