WIP: steps towards better non-python support

I've been experimenting a bit with multi-language support. Changes so far:
* move plugin out of ftplugin/python into plugin/ so that it can be launched from any filetype
* auto-detect languge of kernel and use it to set the filetype/syntax of the shell buffer
* only try to get the pid of the kernel for known languages ( python and julia hardcoded for the moment )

It works at least for executing code and correct syntax highlighting in Python and Julia but I haven't tested doc messages and completion yet. I'm not sure how to best organize language-specific configuration and features,  One idea is refactor the client python code into a base class and let vim-IJulia etc subclass these, but that's probably overkill :)