Add an edn_parse.tag decorator, + docs and tests on tags

[bfontaine]

This adds an edn_format.tag decorator as a shortcut for add_tag:

Before:

def parse_dog(name):
    return {
        "kind": "dog",
        "name": name,
        "message": "woof-woof",
    }

edn_format.add_tag("dog", parse_dog)

After:

@edn_format.tag("dog")
def parse_dog(name):
    return {
        "kind": "dog",
        "name": name,
        "message": "woof-woof",
    }

I also added some docs and tests.

[bfontaine]

Done. I also updated the Contributor’s Note in the README to use twine, as the python setup.py upload command is deprecated.