Use `[[` rather than `$` to extract/subassign options

`[[` is safer, because it won't perform partial matching. Partial matching in the one line that I've edited ended up costing me 5+ hours of debugging, after its revision between knitr 1.5 and 1.6 broke this piece of code: http://stackoverflow.com/questions/11030898/knitr-how-to-align-code-and-plot-side-by-side/23396783#23396783 . (The problem ended up being that the implementation of the newly exposed `code` option ended up capturing -- via partial matching -- the value of my custom `codefig` argument, using that value (TRUE) to overwrite the actual code returned by `unlist(knit_code$get(ref.label), use.names = FALSE)`.)