make sinopia_exporter usable as a command line script

[jmartin-sul]

make this functionality usable from the command-line in a single invocation.

spawned from #1

[jmartin-sul]

indeed, having commander wait on a Promise to resolve does not seem straight-forward. since this takes one and only one parameter at the moment (the group name), and doesn't need to prompt, i'm inclined to just do straight-forward manual parsing of the arg array.

if we did need to prompt, here are links to a useful pattern from the sinopia_acl repo:
https://github.com/LD4P/sinopia_acl/blob/04a3a9cbe260a89aaba5aa73505b560a89f25ab5/src/cli/authenticate.js
https://github.com/LD4P/sinopia_acl/blob/cc373f0100d39ec6a32cffb87b1cba381b5597fa/bin/authenticate