Introduce unity_setup_stubs.h and eliminate UNITY_INCLUDE_SETUP_STUBS.

[jlindgren90]

Requiring a macro like UNITY_INCLUDE_SETUP_STUBS to be defined before
including unity.h is fragile and does not work if unity.h is included
(directly or indirectly) via a command-line directive.

This also resolves issue #438.

[mvandervoord]

I understand the motivation with this one, but I don't love a solution which requires more core files to be includes.

I get that it's almost hypocritical: I spend a lot of time teaching people to break their files into smaller parts and such, but a common complaint that we get with Unity is that it's not all in a single header file. I'd like to not make it MORE files.

I'm not sure I follow the logic in the 2nd statement, that it "does not work if unity.h is included
(directly or indirectly) via a command-line directive." That seems to be a place where it easily DOES work. If UNITY_INCLUDE_SETUP_STUBS is defined at the command line, then it's ALWAYS defined when needed, right? How is that a problem?

I still want to solve the overall problem. I'm moving the conversation back to issue #438 that discusses this.