Support copy to clipboard on code blocks

[TannerYoung]

I searched the issues but was surprised I didn't find one that matched this. Perhaps my search terms were wrong and if so I apologies. Thank you for the wonderful theme :)

Summary

Support a copy-to-clipboard button on code blocks.

Motivation

Improves user experience when interacting with documentation that has bash commands or code which is intended to be copied by the user.

Drawbacks

Potential bloat, as it appears it will need a jquery style solution:

https://stackoverflow.com/questions/48078199/jekyll-code-snippet-copy-to-clipboard-button

[stale]

This issue has been automatically marked as stale because it has not had recent activity.

If this is a bug and you can still reproduce this error on the master branch, please reply with any additional information you have about it in order to keep the issue open.

If this is a feature request, please consider whether it can be accomplished in another way. If it cannot, please elaborate on why it is core to this project and why you feel more than 80% of users would find this beneficial.

This issue will automatically be closed in 7 days if no further activity occurs. Thank you for all your contributions.

[dieghernan]

See #2795