Removing File Extension

Added `file_path_sans_ext` to Pandoc.convert to remove original file extension before adding output extension.  Contributed by Andrew Redd.

This will make pander more consistent with other standards.  It also makes it compatible to integrate into a vignette Engine since according to `?vignetteEngine` 

> Currently the weave function, when operating on a file named 
> ‘\<name\> \<pattern\>’ must produce a file named ‘\<name\>\[\.\](tex|pdf|html)’.