Use a library for argument parsing

[davidzchen]

I think using a library such as getopt or gflags for argument parsing can make the code in the jsonnet tool a lot cleaner. We would probably want to go with getopt since gflags has a slightly different syntax for flags (getopt --long-opt vs gflags --long_opt).

[sparkprime]

Obvious question is do we want to add extra dependencies? I did use getopt in some previous projects and I found it rather obtuse. That's just my opinion though.

By how much would it make the code cleaner?