Add support for a URI command line invocation in addition to a local file

@alisnic: First, thanks for writing this tool. Very useful.

I added a bit of code to support passing a URI via the command line instead of a local file. I've got some dumps in an S3 bucket and it was getting tiresome to download them all the time.

Also noticed that the file/URI was getting reread on each navigation through the stack. Added a return if the presenter has already been computed.