Add argument for cleaning scratch directory after operations

[timwoj]

This came up as part of the rework of zeek-pkg-web. It would be nice if zkg had an option to tell it to delete the files downloaded in to the scratch directory after it is done installing/checking/etc a package. In the context of zeek-pkg-web, we download the package to run the info command but then leave the scratch directory behind once done.

[bbannier]

Is this a subtask or plain dup of #177?

[timwoj]

Is this a subtask or plain dup of #177?

I could go either way. Having it do it automatically without having to call a second command would be nice.

I added it as a sub-issue to #177