Retrieval of Twitter card information

[jez321]

Hi, I was just wondering if it's possible to retrieve Twitter card information such as in the following example html, or if it's a feature you would consider adding (or allow me to submit a PR for)?
Thanks

opengraph/test/html/02_haisai.html

Lines 15 to 16 in ac0f3d5

	<meta name="twitter:card" content="summary"><meta name="twitter:title" content="">
	<meta name="twitter:description" content="All Genre Music Party">

[otiai10]

I tried to secure time to think about the best architecture to implement your point, but couldn't have much time.
If you are still interested in this, could you send a PR and let's discuss on it. @jez321
No pressure.

[otiai10]

og.Extra["twitter"] = YourTwitterImplementation{}

[otiai10]