Add placeholder support for splitting tag by dot

It acts accessing the index which split the tag with '.' (dot).
It will get 'td' by ${tags[0]} and 'apache' by ${tags[1]} when the tag was `td.apache.access`.

#### example

`apache.access` tag will work like below.
* `${tags[0]}` or `__TAGS[0]__` => apache
* `${tags[1]}` or `__TAGS[1]__` => access