Add new BashCompSubdirsInDir annotation

This first `cd` to a specified directory, then
lists the subdirectories therein with `_filedir -d`.

This can be used by e.g. `hugo --theme=[Tab][Tab]`, which would
give a list of subdirectories under the `themes` directory.

bash_completions.go

@@ -13,6 +13,7 @@ import (
 const (
 	BashCompFilenameExt     = "cobra_annotation_bash_completion_filename_extentions"
 	BashCompOneRequiredFlag = "cobra_annotation_bash_completion_one_required_flag"
+	BashCompSubdirsInDir    = "cobra_annotation_bash_completion_subdirs_in_dir"
 )
 
 func preamble(out *bytes.Buffer) {
@@ -100,6 +101,12 @@ __handle_filename_extension_flag()
     _filedir "@(${ext})"
 }
 
+__handle_subdirs_in_dir_flag()
+{
+    local dir="$1"
+    pushd "${dir}" >/dev/null 2>&1 && _filedir -d && popd >/dev/null 2>&1
+}
+
 __handle_flag()
 {
     __debug "${FUNCNAME}: c is $c words[c] is ${words[c]}"
@@ -226,6 +233,16 @@ func writeFlagHandler(name string, annotations map[string][]string, out *bytes.B
 				ext := "_filedir"
 				fmt.Fprintf(out, "    flags_completion+=(%q)\n", ext)
 			}
+		case BashCompSubdirsInDir:
+			fmt.Fprintf(out, "    flags_with_completion+=(%q)\n", name)
+
+			if len(value) == 1 {
+				ext := "__handle_subdirs_in_dir_flag " + value[0]
+				fmt.Fprintf(out, "    flags_completion+=(%q)\n", ext)
+			} else {
+				ext := "_filedir -d"
+				fmt.Fprintf(out, "    flags_completion+=(%q)\n", ext)
+			}
 		}
 	}
 }