Add support for ln=target

[sharkdp]

Quoting from the ls source code:

/* When true, in a color listing, color each symlink name according to the
   type of file it points to.  Otherwise, color them according to the 'ln'
   directive in LS_COLORS.  Dangling (orphan) symlinks are treated specially,
   regardless.  This is set when 'ln=target' appears in LS_COLORS.  */

static bool color_symlink_as_referent;

[sharkdp]

Closing this as "wont fix". We would have to call metadata in addition to symlink_metadata in order to support this. This is not compatible with the current API.