This slightly changes IsAvailableCommand in that a non-runnable command
with a runnable subcommand is now 'Available'
We also use IsAvailableCommand in the rest of the codebase instead of
half kinda sorta doing it incorrectly other places.
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.
We were trying to call a bash function with bash stuff like @ () from a
variable. Stop that. Just call a function with an arg from a variable
instead of trying to pass around the bash.
Should fix https://github.com/spf13/cobra/pull/103
