add optional 'start' param to '{@idx}'

currently, if you want to use `{$idx}` as a 1-based index, you need to do something like this:
```
{@math key="{$idx}" method="add" operand="1" /}
````

with this pull request, the idx helper gets a new optional param `start` which will allow doing something like:
```
{@idx start="1"}{.}{/idx}
```