roam() cleanup

[TimTaylor]

I guess this is similar to #16. After previewing a file with roam() some resources are left behind in the xxx__files directory. Should these be cleaned up?

[yihui]

The tricky thing is that I don't want to accidentally clean up the __files directory that should have been kept. I always feel nervous about deleting files in the user directory (which is why it took a while for me to solve #16---I need to be very careful).

One possible way to solve this problem is to generate plots in tempdir(), where it may be safer to delete files. I'll think more about it.