Allow custom kernel Popen command

reissue of #2853, because it didn't seem to want to track the right commits

- [x] adds `KernelManager.kernel_cmd` configurable for launching a custom kernel
- [x] splits entry_point.base_launch_kernel into two steps: making the launch cmd and launching the subprocess
- [ ] figure out where the entry_point functions belong, if it should be anywhere else

If you use [*my*](https://github.com/minrk/iruby) iruby branch (every other one I found had failures in the msg spec / session), you can run ruby kernels with qtconsole/console/notebook with:

    ipython console --KernelManager.kernel_cmd='["ruby", "lib/kernel.rb", "{connection_file}"]' --Session.key=''

supersedes #2643