To acquiesce to the popularity and diversity of ls alternatives (e.g. https://github.com/ogham/exa, https://github.com/Peltoche/lsd, https://github.com/athityakumar/colorls, tree, all of which have many different options to configure them), I have finally decided to make the command used to preview directories configurable. Users can place their desired command in fzf_preview_dir_cmd and fzf.fish will use it.

Related to #117 and #101