Scribunto parser support

Scribunto parser support

Add $indexOffset parameter to PPFrame::newChild(). This makes it
possible to use newChild() for interpreting named parameters to
invoke in Scribunto -- otherwise I would have had to duplicate its
functionality, which would have been tricky given that I wanted to
make a real frame with an expand() method. Setting $indexOffset allows
newChild() to start counting numbered parameters from somewhere other
than the first pipe character, leaving room for the Scribunto function
name.

Fixed PPCustomFrame_*::getArguments(), was missing for no apparent
reason. I didn't end up using it in Scribunto, but there's no harm in
adding it anyway.

Change-Id: I0c761aab8a7f1ae74e8d151a1346febb5c466e18