Better API ergonomics

[abetkin]

How about

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

For example, it would allow to specify options like

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

Also, we may add the ability to write

import ipynb.finder
import nb1
import nb2

I can make a PR if it's ok