Implement Init and `source` Command

The following changes will allow ion to evaluate an init file in the user's home directory, such as /home/mmstick/.ionrc. In addition, I've also added the capability to use the source command which will either read from a defined file argument as input or re-read the init file. Hopefully you won't mind, but I've also cleaned up the last bit of code at the end of the main function so that it's a while let loop instead of a loop loop.