Add command-line usage

[goodmami]

Some tasks, such as adding a new wordnet, are trivially done via the command line:

$ python -c 'import wn; wn.download("ewn:2020")'

But this isn't very user friendly:

• The -c option to Python may be less well-known than -m
• There's no argparse help for incorrect commands
• Running a query like wn.synsets(...) won't print anything unless the user does print(wn.synsets(...))
• Users must be careful about mixing quotes inside the string

Adding a __main__.py file with a basic command-line interface could help here. I'm not yet sold on making a wn command, as that's the name of the Princeton wordnet utility.

The command-line interface would be convenient with subcommands:

$ python -m wn download ewn:2020
$ python -m wn add ../odenet/odenet/wordnet/deWordNet.xml
$ python -m wn words --lgcode=en cat
$ python -m wn words --lgcode=en cat --translate=ja