Move prex_*() output files to a temp dir

[jennybc]

Several of us have noticed that prex_*() functions now leave their files in current working directory. I'm not completely sure if this is new behaviour, but it doesn't really matter. Consider whether to move these files to a temp dir. On the surface, seems like it should be fine, but I need to analyze it more carefully.

[jennybc]

FWIW I did verify that this is changed behaviour between v1.0.0 and v2.0.0. Root cause is the location of input (below temp dir in v1.0.0 vs. in working directory in v2.0.0). We need to be cool with writing the reprex file to one place (below temp dir), even though we're going to tell knitr to work in current working directory.