FIX: Retain parent FOR loop's $idx variable in IF command content

[deedw]

As described in the README, the IF command is implemented using a FOR loop with 1 or 0 iterations.

A side-effect of this is that the $idx variable is also reset to 0 within the IF command content. For example:

+++FOR value IN ['a', 'b', 'c'] +++
+++INS $idx +++
+++IF value==='c' +++
+++INS $idx +++
+++END-IF +++
+++END-FOR value +++

will generate:

0
1
2
0

when expected output would be:

0
1
2
2

This pull request changes the behaviour so that the output is as expected and the innermost FOR loop $idx value is retained.