Allow arbitrary url

The default is to install the JSON from
`http://www.unicode.org/Public/cldr/<version>/json.zip`.

    npm install cldr-data

In order to install the full JSON
`http://www.unicode.org/Public/cldr/<version>/json-full.zip`, use:

    CLDR_URL_KEY="json-full" npm install cldr-data

In order to pass a different url host and path, use:

    CLDR_URL="http://foo.bar/my/path/to/Public/cldr" npm install cldr-data

Fixes #2
26.0.3