Revert "Automatically add STEX extension to otherwise unused include files"

I guess I just shouldn't commit things when I'm sick...

This reverts commit 7e81b9be2a661648cf27478105701bbce6f6c0b4.
Revert "Have texpp append .stex when it's not already there."

I guess I just shouldn't commit things when I'm sick...

This reverts commit d7bfe194bbf21520ff9ae6f7b777072e49c576b8.
Add some test cases for tex{pp,strip}

This should prevent me from causing any trouble when I'm sick... :)
Actually support "\input{FILE.tex}" and "\input{FILE}"

I have no idea what I was doing yesterday, but this actually works (as
opposed to the other one, which clearly didn't work at all).  This
also has the advantage of supplying some tests, which helps a lot!