Add `:reload` Support

[Thomas Mattacchione]

Is your feature request related to a problem? Please describe.

I would like to run nbb from a JS program which is itself running in a "watch loop". The blocker is that dependencies of the nbb scripts won't reload when changes are made to them.

Describe the solution you'd like

I'm not opinionated about the solution, but it sounds like adding support for :reload may resolve this issue.

Describe alternatives you've considered

nbb seeing a file was modified and reloading the namespace accordingly (but this is a naive approach and may not be the responsibility of nbb itself)

Additional context

see eleventy-test-cljs for a "minimal" repro of the problem. Also refer to the original slack thread.