This change is a simplified implementation of the Builder.Path() and
Visitor().ExpandPathsToFileVisitors() functions used by kubectl to parse files
and directories. The filepath.Walk() function is used to recursively traverse
directories. Every .yaml or .json resource file in the directory is read
into its own io.Reader. All the readers are then passed to the YAMLDecoder in the
InjectYAML() function.
Fixes#1376
Signed-off-by: ihcsim <ihcsim@gmail.com>