Support for Explicit Output of HC Defaults

[hcpchris]

In v.1.0.0, Highcharts Core for Python outputs the JavaScript literal object where any unspecified property is serialized to a dict as None and subsequently stripped from the serialized JS object literal version. Logically, this will then apply the Highcharts default value for that property.

In v.1.x, it would be nice (to aid in debugging / diagnostics) to have an show_default_values argument in the .to_js_literal() method which if True will output the JS literal with those default values explicitly displayed, rather than stripped. By default, this argument should be False.