Add `tabsAttributes` to plugin options

[starkwang]

This change is to add tabsAttributes to global plugin option. It would be useful when we want to config the default tabs attributes globally.

For example:

// .vuepress/config.js
module.exports = {
    plugins: [
        [
            'vuepress-plugin-tabs',
            {
                tabsAttributes: {
                    options: {
                        useUrlFragment: false
                    }
                }
            }
        ]
    ]
}

Then all the <tabs> components will have :options="{useUrlFragment: false}" attribute by default.