readline fromtend for ipython 5.4+ and 6.0+

See ipython/ipython#10364 for information.

This code has been originally extracted from IPython 5.x-dev, so is under IPython's LICENSE.

This is something that we offer in support of historical compatibility and certain specific use cases where our main interface (prompt-toolkit) isn't optimal. But we do not envision any significant development beyond fixing critical bugs. We only have the resources to offer this as a best-effort solution. If you are interested in doing any significant development on this tool, please let us know via an issue, and we can explore transferring maintainership of the package to you.