REPL Autoload file

This PR enables users to specify commands that will automatically be executed when the REPL loads (such as including time zone, sources, etc).

So far, I have the repl loading a single file in the user's home directory (~/.scalding_repl).

Is there any value in trying to make it search other places for files, such as walking up to the root from the current directory (as many other configs do). This doesn't seem particularly useful for the way most people run from the root of the project.