Table id including date; for table rotation by date

This patch makes `table` and `tables` options accept [Time#strftime](http://ruby-doc.org/core-1.9.3/Time.html#method-i-strftime) format, so that the table id can include date. It enables table rotation by date.

Letters which can be used in table ids ([a-zA-Z0-9_]) have no special meaning in Time#strftime format. Therefore, the patch does not break existent settings.
Use `Fluent::Engine.now`, which provides current timestamp object in Fluentd runtime.