add placeholder support for splitting tag by dot

The placeholder of {$tag_parts[n]} and __TAG_PARTS[n]__ acts accessing the index which split the tag with "." (dot).
For example with td.apache.access tag, it will get td by ${tag_parts[0]} and apache by ${tag_parts[1]}.
improve placeholder error handling
Merge pull request #9 from y-ken/feature/split_tag_by_dot_placeholder

Add placeholder support for splitting tag by dot