Add html_fragment format

Okay, I'll take the PR as-is and then make a change to RStudio to fix the
previewing issue.


On Mon, Jul 28, 2014 at 4:00 PM, Hadley Wickham <notifications@github.com>
wrote:

> In inst/templates/fragment.html:
>
> > @@ -0,0 +1,2 @@
> > +<meta charset="utf-8">
>
> Yeah, I'm not sure how people will get this into where they need it - I've
> been copying and pasting.
>
> I agree that it would be nice to fix the previewing problems separately.
>
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> <https://github.com/rstudio/rmarkdown/pull/192/files#r15486504>.
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