C++ is not currently supported

Don't pretend that it is
Trimmed some comments I forgot earlier
Some preparation for adding testing

I'm planning on having a separate test target, and to avoid code
duplication, I'd like to have it decide using a preprocesser flag. So I
need to change the extension and compile it using gcc
Added configs in preparation for test build
Fixed a small typo
Moved the configs as the tup man pages suggest

It helps prevent clutter for the newcomer and seems reasonable.
Updated the README to accomodate the build changes

It now discusses the two different build targets
Added initial test framework

I'm unsure of this design. And, more importantly, some of the details
about how the actual test runner will look. But it seems like a
reasonable place to start so I'll run with it for now. Also, you might
notice that test_kernel_early prints out a message that is immediately
wiped out by the kernel_early. I know. I'm thinking of having the kernel
halt on a test failure so you can see the details of the test, but
that's one of the aspects I have to decide on.