Provide a template function to instantiate a kernel

[JohanMabille]

The different xeus-xxx kernels instantiate the kernl object the same way, leading to redundant code in these projects. One way to reduce this boilerplate would be to provide a template function for instantiating a kernel object.

[SylvainCorlay]

Yes, the least duplication between xeus-* kernels the better!
This could also go in the cookiecutter.

[AntoinePrv]

This could also go in the cookiecutter.

I think we should avoid the cookiecutter as much as possible since it does not give an easy way for the kernels to update to new versions. It's also harder to maintain jinja templated code.