Allow custom In/Out prompts

Feature request: it should be possible for a Jupyter kernel to change the `In[ ]` and `Out[ ]` prompts. It would make sense to have prompts which are the same as the command-line application corresponding to the kernel. This is especially true in the case that history is supported, but with a different syntax than `Out[n]`.

I suggest this is implemented by introducing an extra field in the kernel spec.