Better API ergonomics

How about

```python
with ipynb.importer(full=True): # or defs=True
import my_notebook
```

For example, it would allow to specify options like

```python
with ipynb.importer(ignore_errors=True):
import my_notebook
```

Also, we may add the ability to write

```python
import ipynb.finder
import nb1
import nb2
```

I can make a PR if it's ok