Add litedown support

[dmurdoch]

litedown is Yihui's lightweight version of rmarkdown + knitr. Whereas there was support for htmlwidgets in knitr, there's none in litedown. This PR adds it.

It does require a couple of small changes to litedown which have been submitted: yihui/litedown#86 .

[jcheng5]

Thanks @dmurdoch! I’m fine with this change in spirit but would prefer to wait until the litedown change is merged.

[dmurdoch]

That makes sense. I just realized one thing: if there's an htmlwidget that uses an htmlDependency sourced online, it won't work. I can see how it should be fixed, and will modify soon.

[dmurdoch]

I've just added support for href dependencies. I am not sure I understand all the possible forms of src in a dependency, and the code doesn't handle them all, but I was only able to find one package that used href, and none of the more exotic possibilities.