add options to cull named servers and default servers separately

[HCharlie]

addressing issue, add options to cull named servers only and default servers only

[welcome[bot]]

Thanks for submitting your first pull request! You are awesome! :hugs:
If you haven't done so already, check out Jupyter's Code of Conduct. You can meet the other Jovyans by joining our Discourse forum. There is also a intro thread there where you can stop by and say Hi! :wave:
Welcome to the Jupyter community! :tada:

[yuvipanda]

Apologies for the late review, @HCharlie! I think this looks good, I made a minor change to help clarify the code a little more. Going to merge now.