Fixing case where duo treated folder arguments as sub-commands

When the first argument passed is a folder-name, duo was trying to treat it like a subcommand, and then would fail. For example:

```sh
$ duo public

    error : duo-public(1) does not exist
```

I've changed the sub-command check to handle this new use-case as well, where that conditional looks like `command && !extname(command) && !exists(command)`

I opted for the extname check to avoid a sync call to the filesystem if we were looking at something like `index.js`, but if we wanted to support `.` in sub-command names, this will have to change to something like: `command && !isFile(command) && !isDir(command)`

The point is, all the tests pass :)