Added security section and nicely wrapped lines
Try harder to find .install file

I was installing zsh-syntax-highlighting and realized that it printed
out stuff from a .install file when spinach hadn't let me check it. Now
it warns when it can't find it, and it'll look for *.install to open or
print out the directory for you to look through manually.
Added aur3.org warning and made -u work with it

It looks like the mirror isn't being updated though, so I may eventually
just remove this functionality to shrink the script a bit. I suppose it
may be useful as a back-in-time install sort of thing.