[FEATURE] added v:iterator.for, improved v:iterator.loop closes #350

Hey,

Added that ``v:iterator.for`` as discussed, closes #350.

I also improved ``v:iterator.loop`` and added ``minimum`` and ``maximum`` usage.

I also fixed a bug in ``v:iterator.loop`` where the check for a existing ``iteration`` variable always used the default value.

It didn't max sense to implement ``step`` for ``v:iterator.loop`` though as that would confuse the usage of ``minimum`` and ``maximum``