Integrate with Cmd objects

[zot]

This provides a @g_str macro that creates a Cmd object based on the interpolated contents.
This allows absolute paths and uses pwd for the directory if the glob not absolute.

If these files exist: /tmp/a1, /tmp/a2, and /tmp/a3,
then:

julia> import Glob.@g_str

julia> x="a"
"a"

julia> g"/tmp/$x*"
`/tmp/a1 /tmp/a2 /tmp/a3`