Command line interface to manage kernel specs

This is part of [IPEP 25](https://github.com/ipython/ipython/wiki/IPEP-25:-Registry-of-installed-kernels), the kernel registry. This PR adds:

- Python API to install kernel specs
- Command line interface to install kernel specs - `ipython kernelspec install path/to/dir`
- CLI to list installed kernels - `ipython kernelspec list`