Fix ftdetect macro

[Nicop06]

Since vim v7.4.365, all *.Rmd files have the filetype rmd by default while this
plugin is expecting rmarkdown filetype. Using set filetype= instead of
setf somehow gives the autocmd macro a higher priority and is not
overwritten by vim default settings. I tested on vim 8.0.134 only.

This is only a workaround and it does not solve the underlying issue. The best
solution would be to use the rmd filetype directly, but this requires more
changes.

[fmoralesc]

Good to know. Thanks for the patch.