Define a render/HTML module and protocol

[wulymammoth]

If we follow what the other Prismic SDK's provide out of the box, we should provide a way to render mark-up.

We can absolve each fragment of rendering responsibilities by creating a module whose responsibility is to take a fragment struct and rendering safe HTML string with it.

Alternatively, we could also just implement the Phoenix.HTML protocol for each fragment :)

[newtrat]

I'm gonna close this issue because I think we've usually found that it's easier to specify the HTML structure in the caller. Prismic doesn't really dictate a presentation for the data you put in it, so I'm not sure it makes sense to impose one in our library, even if it's purely optional.

[newtrat]

Sorry, I just realized we have dummy as_html implementations all over the codebase. Should probably get rid of those if we're serious about never rendering HTML. So I'll keep this issue open for now.