clean_gallery: clean out snippet output from a gallery
In dgrtwo/rgallery: Build a Gallery of R Snippets

Clean out the .md files, along with image and cache directories, from an R-gallery. This doesn't remove the files in the _R directory itself.

1 2	clean_gallery(gallery, infolder = "_R", outfolder = "snippets", images_dir = "images", caches_dir = "_caches")

`gallery`	directory of the gallery to be cleaned
`infolder`	input directory
`outfolder`	directory
`images_dir`	directory to store images
`caches_dir`	directory to store caches This will not remove a .md file, image directory, or cache directory that does not have a corresponding .Rmd file in the input (it will show a message).