Restrict copy-paste to input code

[hturner]

This comment provides some code which allows you to show code input and out, but restrict the copy-to-clipboard function to just the input code.

This could be worth exploring as an improvement to the docs.

[hturner]

This idea was investigated as part of an R hackathon, where the conclusion was that this approach is not such a good idea.

A better solution may be to be more careful with when we use an ```R block (that people are likely to want to copy and run) vs an ```Rconsole block (that people can still copy all/part of, but is mainly displaying input/output for reference). I'll open a separate issue about that.