Schemes with line highlighting

First of all: I *really* like Perv Orange.

But I was angry that I have to modify Perv Orange themes to get line highlighting the way it should be. (And you know that editing these files isn't easy when installed from Package Control.)

So I created two files (both derive from existing ones) but with line highlighting turned on.

And I simplified comments regarding line highlighting in existing themes.

And changed `README.md` accordingly.