Option to run in global namespace

[jayqi]

This goes against the philosophy of reproducible examples, and would therefore be discouraged. However, it could still be potentially be handy to create a reprex that ran in the global namespace.

One potential use case is someone working in a Jupyter notebook, who wants to quickly share a snippet of code and output that is not meant to be runnable. They could run the cell magic and evaluate the code in notebook's namespace.