grovel: make INVOKE return the command output
grovel: make INVOKE return the command output

(gh issue #50)

attila-lendvai/cffi,stassats/cffi,stassats/cffi,mcna/cffi,3b/cffi,cffi/cffi,3b/cffi,mcna/cffi,3b/cffi,cffi/cffi,dkochmanski/cffi,stassats/cffi,attila-lendvai/cffi,dkochmanski/cffi,cffi/cffi,dkochmanski/cffi