Allow variables in external panelize command

[sjmurdoch]

Currently, variables (e.g. %f, %d) are expanded on the command line, extension files and the user menu. It would be great if they also could be used in the external panelize command. I am particularly interested in the %{...} variable so one of the stored external panelize commands could be parameterized by a string input by the user. This functionality could be used with git grep -l %{search} to list files containing a particular string.