Support for YouTube embed flags

[chris13524]

In the README, it says the plugins supports various URL forms, such as "XKFDS?rel=0". However, the plugin ignores these flags when it generates the iframe embed.

I suggest there be some mechanism to use these flags. Directly in the URL would probably be the easiest as it would "only" involve modifying the regex to include these flags in the URL.

@[youtube](https://www.youtube.com/embed/KMU0tzLwhbE?rel=0)
@[youtube](KMU0tzLwhbE?rel=0)
@[youtube](<iframe width="560" height="315" src="https://www.youtube.com/embed/KMU0tzLwhbE?rel=0" frameborder="0" allow="autoplay; encrypted-media" allowfullscreen></iframe>)

Another possibility would be to put the flags inside the square brackets. This would be much more readable than the first option, but wouldn't allow directly pasting the embed code.

@[youtube:nosuggest](KMU0tzLwhbE)

A third choice would be to add an entry in the options to modify this behavior globally. But this of course prevents modification of the flags on a case-by-case basis.

var md = require('markdown-it')({
  html: true,
  linkify: true,
  typography: true
}).use(require('markdown-it-video', { // <-- this use(package_name) is required
  youtube: { width: 640, height: 390, suggest: false },
}));

I think the best way would be to support all 3! With the precedence [options](..) -> [..](..?options) -> global