add time_default_current (default true) to disable filling current time when time is not produced by parser result
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
Fix feature from config_param to attr_accessor
 * not to be enabled on plugin direct use
for clear code
add comment for implement restrictions
fix configuration parameter name for readability
fix tests with current code
Merge pull request #370 from tagomoris/parser_default_current_time

Add "estimate_current_event" attribute to TextParsers not to set current time into parse result