export and document pandoc_to() and pandoc_from()

[cderv]

this closes #1950

And I took the opportunity to start changing styles of tests.

[yihui]

I'd merge the doc of these functions to

#' @rdname output_type

since they are essentially the same thing.

[cderv]

This is what I made at first but think back and document in its home page. I have seen that as lower level and I thought it would be interesting to have them link (by See also) but separate.

I should have stick with my first feeling.

[yihui]

Thanks!