Add offline mode

[koddsson]

• The component should render with the given slots.
• The component should not make a network call (excluding images).

See branch: https://github.com/WebComponentsGuide/toot-embed/tree/complete

[koddsson]

This could render the data that's provided and make a network call and then render all the missing data after the network call.

[koddsson]

• The component should not make a network call (excluding images).

We should also make the component render blurhash for all the images.