allow addition of non-repo paths for gita shell command

[nosarthur]

I often find the need to run bash command in another non-repo folder; this could be achieved by

gita shell <folder-alias> <shell-command>

if non-repo paths can be added.

We can add a -d option to gita add to allow such addition. And the corresponding path(s) will be saved in folders.csv config with similar format as repos.csv