Fall back to _filedir, compgen -f, etc. if no matches are returned

Programs that use omelette completion can no longer complete files and directories. I modified my completion script to fall back to `_filedir` (it relies on the `$cur` and `$prev` variables):

``` bash
_p_complette() {
local cur prev

cur=${COMP_WORDS[COMP_CWORD]}
prev=${COMP_WORDS[COMP_CWORD-1]}

COMPREPLY=( $(compgen -W '$(projects --compbash --compgen "${COMP_CWORD}" "${prev}" "${COMP_LINE}")' -- "$cur") )

[[ $COMPREPLY ]] && return

_filedir
}
complete -F _p_complette p
```

In the same way `compgen -f` with `compopt -o filenames` could be used, I think.