feat: allow external control

[pReya]

closes #56

Currently the possibilities to control the iframe from the outside were a little limited (and not well documented). So I fixed this by:

• Adding enableJsApi prop, which tells the YT iframe to listen for messages
• Adding alwaysLoadIframe prop, in case someone wants to externally start playing the video (without clicking the poster first)
• Adding more documentation on controlling the iframe from the outside
• Refactored the creation of the YT iframe parameter string (using the URLSearchParams() class instead of manual template string fiddling) -> this should be less error prone
• Ran prettier on everything

[pReya]

@ibrahimcesar Can you give this a review?

[ibrahimcesar]

LGTM!