Implement "embedded" command to extract static resources (#9982)

* draft

* Implement extract command

* Fix nits and force args on extract

* Add !bindata stub, support Windows, fmt

* fix vendored flag

* Remove leading slash for matching

* Add docs

* Fix typos

* Add embedded view command

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

modules/templates/static.go

@@ -229,3 +229,16 @@ func AssetNames() []string {
 	}
 	return results
 }
+
+func AssetIsDir(name string) (bool, error) {
+	if f, err := Assets.Open("/" + name); err != nil {
+		return false, err
+	} else {
+		defer f.Close()
+		if fi, err := f.Stat(); err != nil {
+			return false, err
+		} else {
+			return fi.IsDir(), nil
+		}
+	}
+}