Enable autoplay by default

[alistair3149]

When the user interact with the load content button, it is likely that they are intended to play the video. By making autoplay the default option, user can avoid needing to manually click the play button after the embed is loaded, making the interaction more seamless. It is especially helpful with #30 since it will likely introduce more interaction.

Suggestions:

• Make autoplay behavior consistent across services (e.g. Twitch autoplays by default currently)
• Add global configuration to change the autoplay default
• Add local override to the default in the parser tag