add theme API & configuration

[mbektas]

• allows setting theme from CLI with --theme option. Examples: --theme dark (same as --theme "JupyterLab Dark"), --theme light (same as --theme "JupyterLab Light")
• adds theme namespace to API with functions to interact with theme

fixes #30

[jtpio]

Thanks!

[martinRenou]

Amazing! Thanks @mbektas

[jtpio]

@mbektas did you want to release a new version with this change?

Otherwise I'm happy to do it. For that particular release we can probably just bump to 3.0.7-2 and publish.

It would also be a good opportunity to address #34

[mbektas]

@mbektas did you want to release a new version with this change?

Otherwise I'm happy to do it. For that particular release we can probably just bump to 3.0.7-2 and publish.

It would also be a good opportunity to address #34

@jtpio, I will publish now, next time feel free to publish if I forget / am late to do it. 3.0.7-2 is the proper version as you suggested.

[jtpio]

Thanks!