Optionally reject on invalid URL

[bryanburgers]

ExpressionEngine has a validation stage that allows a plugin to reject values. We should use this to reject invalid URLs.

However, for backwards compatibility, we can't just start doing this. So, there should be an option when setting up the fieldtype.

[bryanburgers]

Would this require actually hitting the Vimeo and YouTube servers to check that the key is valid? Or can we just pass some information from the JavaScript validation?

Checking on the server would be more correct. Checking that the JavaScript validation passed would be far easier.