Add command-line arguments

[rlafuente]

Some ideas (from easiest to implement to hardest):

• Overwrite existing datapackage.json
• Validate existing datapackage.json using the datapackage module
• Non-interactive mode, where all values are guessed, inferred or blank. For when packagers are in a hurry.

[svHatch]

In addition to that, what about a template argument, which could provide defaults as per your 3rd point. The template would be an existing datapackage.json