add start_at option to youtube directive

[marianoguerra]

Requested Feature: add an extra option to the youtube directive to specify where the video should start at

Related Area: plugins

Do you want to contribute this yourself as a pull request?

• [x] Yes, I have already written code for it (link if available and feasible)
• [ ] Yes, I don’t have code ready yet (that’s okay!)
• [ ] No (that’s okay too!)

Does this feature affect backwards compatibility? If yes, in what way? no, it's an extra option that doesn't affect existing uses of the directive

Rationale and full description: it's common to embed a youtube video at a particular time mark

[Kwpolska]

Solved by #3604.