Install option of the downloaded package

@whimboo
@davehunt

Hi,

I had this idea earlier today about adding a flag to let a download be directly installed in one go using mozinstall. I had to do this a couple of times before, so I thought it might be useful and I wanted to see if I can do it.

Let me know if it is useful.


NOTE: The flag is a bool, because I cannot pass optional string values with optparse. If we ever switch to argparse, I would by default try and use the directory of download, but leave the option of specifying a different directory open to the user.