Implement BASH_DIRTRIM_TEXT

[Mustafa Mohamad]

See for example 
https://github.com/zaufi/paludis-autopatches/blob/master/ebuild_unpack_post/app-shells/bash/bash-4.2-unicode-triple-dot-trim-path.patch

Which replace the default '...' text w/ a single unicode character '…'

I propose adding such an option in future version to make it much easier to 
change this.