[Suggestion] Filtered Temp Dir

A common pattern I see is using `with-pre-wrap` and then copying some files to a tmp dir which is then added back to the fileset.

It would be nice if we could simplify this a bit with something like the following:

``` clojure
(with-tempfs fileset (by-ext ["hl"])
... ;;other things
)
```

Which would wrap `with-pre-wrap`, the file copy and `(-> fileset add-resource commit!)`, and a `*tmp*` var available to the user for accessing the temp files.

Thoughts?