Added omnisharp-jack-in function

This is a simple interactive function to start the OmniSharp server process. It's like the `cider-jack-in` function of the Cider/nREPL Emacs package.

When called, this function prompts the user for the solution files path and then starts the server process with the given solution. If I need to change the name of this function, I'm perfectly fine with that.