Add support for Beaker Notebook

[vlasenkov]

I'm trying to use TQDM in Beaker Notebook.

from tqdm import tqdm_notebook
import time
for i in tqdm_notebook(range(10)):
    time.sleep(1)

Produces:

Widget Javascript not detected.  It may not be installed properly. Did you enable the widgetsnbextension? If not, then run "jupyter nbextension enable --py --sys-prefix widgetsnbextension"

At the same time it's ok in Jupyter.
Would be great to add support for Beaker.

[casperdcl]

@vlasenkov the real maintained repo is at https://github.com/tqdm/tqdm.