add asis, other output arguments

Thanks for merging in the last request! This one adds a few additional options beyond `hide=true`: `asis=true`, `commands=false`, `results=false` and `output=false`. (This is kinda like knitr's `echo=FALSE` and `results="asis"`, but a bit simpler.) It also exports a method `gadfly_format` which is basically `string`, but prints vectors like they used to be, and allows other users to modify the output of objects by importing. (Useful with `asis=true`.) Finally, the flushing of `WeaveSandbox.MIME_OUTPUT` in `execblock_julia` is for some reason needed, as I was getting two Gadfly plots for the price of one.