Update plugin options interface

[samuelmaddock]

The current plugin options are inflexible and will possibly lead to breaking changes in the future:

new ThemePlugin({
    themes: {
        light: 'light.css',
        dark: 'dark.css'
    }
})

Using an array for themes would be more flexible:

new ThemePlugin({
    themes: [
        {name: 'light', file: 'light.css'}
        {name: 'dark', file: 'dark.css'}
    ]
})