Add support for 'nth' based tpls

Two new parameters:
&nth
Target any thumb with an 'idx' divisible by '&nth'.

&nthTpl
Name of a chunk serving as template for every '&nth' thumb.

Please note - I've also set the '$idx' property to start at 1 instead of 0. Have not seen any side effects but let me know if this could cause an issue.