Add --verbose and --debug flags for choosing which kinds of messages to show to the screen

[tarmstrong]

Right now, a bunch of stuff gets printed to the screen when running nbdiff and nbmerge, namely, Flask messages showing the status of every HTTP request. I propose two flags:

• --verbose: show the output of flask HTTP statuses
• --debug: turn on Flask debugging. This adds a few quirks, but it's useful when developing.