role for inline execution

[akhmerov]

Sometimes it's helpful to substitute code evaluation results inside text, so we may consider defining a role jupyter-execute to accompany a directive. The role would obviously have no configuration, probably wouldn't show the source, and probably would only support latex or plaintext output.

[choldgraf]

Note that https://github.com/jupyter/jupyter-sphinx/pull/130 takes a step closer to this, by allowing for inline nodes from cell outputs 👍