Add short option for only-dirs

src/options/flags.rs

@@ -30,7 +30,7 @@ pub static SORT:        Arg = Arg { short: Some(b's'), long: "sort",        take
 pub static IGNORE_GLOB: Arg = Arg { short: Some(b'I'), long: "ignore-glob", takes_value: TakesValue::Necessary(None) };
 pub static GIT_IGNORE:  Arg = Arg { short: None, long: "git-ignore",           takes_value: TakesValue::Forbidden };
 pub static DIRS_FIRST:  Arg = Arg { short: None, long: "group-directories-first",  takes_value: TakesValue::Forbidden };
-pub static ONLY_DIRS:   Arg = Arg { short: None, long: "only-dirs", takes_value: TakesValue::Forbidden };
+pub static ONLY_DIRS:   Arg = Arg { short: Some(b'D'), long: "only-dirs", takes_value: TakesValue::Forbidden };
 const SORTS: Values = &[ "name", "Name", "size", "extension",
                              "Extension", "modified", "accessed",
                              "created", "inode", "type", "none" ];