[bash-completion] Add _fzf_setup_completion to enable fuzzy completion

While we can attach `_fzf_path_completion` or `_fzf_dir_completion` to
any command using the standard bash complete command, the functionality
of the existing completion function is lost.

Use _fzf_setup_completion if you want to extend the existing function
with fuzzy completion instead of completely replacing it.

e.g. _fzf_setup_completion path kubectl