Add getters for og: values

[jeyj0]

setOGImage and similar functions exist, and the meta tags themselves can be rendered using ogImage for example. But Twitter for example could use twitter:image, which can't easily be set to the same value as og:image. The context can be, but having a simple getOGImage and similar functions would make it a lot easier (and getting it from context is not trivial, as there's no documentation that that's where the info is stored).