Allow customization of nob_log

[bynect]

It would be nice to be able to modify the logging output without patching the nob.h source code. The most painless ways that I can come up with are:

• gating the definition of nob_log with NOB_NO_LOG, so the user must provide a nob_log() function themselves
• making nob_log a macro that users can override before including nob.h

This would be useful for example for colored output