Add parts_range_separator

This variable used in accessing `__TAG_PARTS__` with range index. Instead of joining with `.`, it will join with `parts_range_separator`, which defaults to `.`.

Hence, with tag `td.apache.access` and `parts_range_separator` equal to `_`, the result for __TAG_PARTS__[1..-1] will be apache_access.
It will be useful if people want to use parts of tags as folder name, by setting value of `parts_range_separator` with `/`