This first `cd` to a specified directory, then
lists the subdirectories therein with `_filedir -d`.
This can be used by e.g. `hugo --theme=[Tab][Tab]`, which would
give a list of subdirectories under the `themes` directory.
A corner case exists where c.Runnable() is not checked
before c.Run() is called, thus a nil c.Run is executed
leading to "panic: runtime error: invalid memory address
or nil pointer dereference".
This patch adds an extra c.Runnable() check in execute()
to catch that corner case.
Fixes#37.
Anthony Fok