checks/magic: index arguments from end

[proppy]

Fixes #186

[RTimothyEdwards]

Makes sense for the way the script is called on the magic command line from the python script; the arguments passed to the Tcl script will always be at the end, and any arguments to magic will be at the beginning. This is unlikely to change, but I agree that counting from the end is a bit more robust.

[marwaneltoukhy]

@RTimothyEdwards Is this still correct? Shall we merge it?

[RTimothyEdwards]

@marwaneltoukhy : Yes, I think it is still relevant and as I said above it makes more sense for the arguments to be counted from the end backwards. Merging it should not affect script function.