Add always-return-ns-form option to clean-ns

[DerGuteMoritz]

As discussed via Slack earlier today.

Aside: Looks like the "Code inlining with mranderson" mentioned in the checklist is outdated - there's no build.sh (anymore). Instead, I successfully ran make install which seems to be equivalent. Maybe this needs to be updated?

• [X] The commits are consistent with our contribution guidelines
• [X] You've added tests (if possible) to cover your change(s)
• [X] All tests are passing (run lein do clean, test)
• [x] Code inlining with mranderson works and tests pass with inlined code (run ./build.sh install -- takes a long time)
• [X] You've updated the changelog (if adding/changing user-visible functionality)
• [X] You've updated the readme (if adding/changing user-visible functionality)

[vemv]

Thanks! Perfect PR, kudos.

Yes, please add a commit removing that checkbox.

[vemv]

Cheers. I'll cut a release soon, but I take it that the make install cuts it for now