Move assert failure to clar__fail

I frequently want to put a breakpoint on a Clar assertion failure.
This makes it much easier to simply break on `clar__fail` instead
of having to break after testing condition in `clar__assert`.
Added annotated versions of clar_assert_equal...

This adds cl_assert_equal_s_ and cl_assert_equal_i_ which allow
for an extra annotation parameter when using these assertions.
I find that I sometimes need to append a little extra data to a
series of numeric assertions to understand exactly where the error
originated.
Update docs for more current usage info

This is a quick take at updating README.md for a slightly more
current description of how to use Clar.  This isn't great, but it
is more accurate than what's there now.

Also add the minimum `main.c` as `test/main.c.sample` so folks
can have a very "quick start" when setting up.
Merge pull request #34 from arrbee/assertion-failure-function

A few Clar changes that I've been playing with