Add command-line options for template and static file dirs

I had to implement this to use InstantRst with a custom template. 
Also, tried to keep the changes to a minimum, so one still has to add the socket.io client-side code etc. to the template for it to work.
In case you want to accept/merge this, I'll send a pull request for the related changes to https://github.com/Rykka/InstantRst as well.