Set context

[alecdibble]

I'd like to be able to set the context with something like `na use my_namespace` and then be able to run an alias in that namespace with a simple `na my_alias` rather than needing to supply the namespace with every command.

From https://www.reddit.com/r/node/comments/atlqt2/i_built_this_tool_to_solve_my_biggest_frustration/ehcneia/?context=3