Add necessary metadata for editable labextension installs.

[ian-r-rose]

Fixes #177 .

This add some (poorly documented) metadata to the __init__.py which is needed for editable installs to work for the frontend code. @jacobtomlinson would you mind taking this for a spin to see if it fixes the issue for you?

I was able to get things working as expected with

pip install -e .   # editable install for the python code
jupyter labextension develop --overwrite    # editable install for the frontend code

[jacobtomlinson]

That did it! Thanks @ian-r-rose.

I can confirm that if I tweak my local source, run jlpm build and then clear my cache and refresh Jupyter Lab I see the changes.

[jacobtomlinson]

I notice the command in the README is slightly different from the one you commented above. Will this make a difference?

+ jupyter labextension develop --overwrite
- jupyter labextension develop . --overwrite

[ian-r-rose]

Thanks for checking @jacobtomlinson!

I notice the command in the README is slightly different from the one you commented above. Will this make a difference?

+ jupyter labextension develop --overwrite
- jupyter labextension develop . --overwrite

I believe both should be fine (it defaults to .).