User preferences replace defaults, instead of extending

[plutonium-239]

If I add a custom rule to the user settings:

{
    "tags": {
        "Important2": {
            "scope": "comments.important",
            "identifier": "!+",
            "underline": false,
            "stippled_underline": false,
            "squiggly_underline": false,
            "outline": false,
            "is_regex": true,
            "ignorecase": true,
        }
    }
}

Then the plugin only considers Important2 to be highlighted, and all the default TODO, Important, FIXME etc. don't.

As a workaround, I have to copy over each of the defined defaults to my user settings, which is unexpected.

This is probably because of plugin/settings.py#L171

It should be changed to include the default first and then update it with the user tags.