This change improves the execution speed by reducing the allocation count (not allocating a main output buffer anymore for template generation when using Template.ExecuteWriter). Instead, the function directly writes to the given io.Writer. This behavior is now default when using ExecuteWriter(); therefore I've removed ExecuteBuffer() again whose goal was the same. Updates #63 and fixes #65.
Backwards-incompatible API-Change  for custom tags: tag execution functions are not taking a buffer anymore; instead, they are taking the new TemplateWriter interface.