Add config_param "time_default_current" not to set current time into parse result

 * By setting current time, caller cannot know "time" is from whether parse result or current time
   * it brings troubles especially in case for filter plugins like fluent-plugin-parser
 * naming problem:
   * 'time_' prefix seems important for me

```rb
if value.nil? && @time_default_current
elsif value.nil?
...
```

seems dupilcated conditions.
I like following code to be clear:

```rb
if value.nil?
  if @time_default_current
  else
  end
else
```