Pagenumbers aren't defined

If you make a multipage document, the pagenumbers will be questionmarks.
This can be resolved by a change in `jupyterlab_latex/build.py`
Line 197 and 198 should be changed into:
```
whitelist=[tex_base_name+'.pdf', tex_base_name+'.synctex.gz', tex_base_name+'.aux', tex_base_name+'.toc'],
greylist=[]
```
