a potential bug: because list[[i]] = NULL will remove i from list

a safer way is list[i] = list(NULL); why bother? because otherwise out.width/out.height will be removed if NULL, and I added out.width.px/out.height.px in options, then due to R's evil partial matching, when out.width is removed, options$out.width will fetch options$out.width.px; this is the reason of the previous failure on Travis CI
no need to carry over the name attribute
cosmetic