Implement different levels of "make clean"

[ezzatron]

In previous implementations of the makefiles, we used to have make clean and make clean-all. A similar concept might be useful here.

Two things stand out as being potential candidates to not remove under the standard make clean target:

• Dependencies (e.g. node_modules / vendor directories)
• The .makefiles directory itself

Implementing this might solve #36, which we currently think is caused by the .makefiles directory being removed before some of the double-colon clean:: targets run.

The question is: what to call these different targets, and what should be cleaned by each?

[ezzatron]

@jmalloc FYI

[jmalloc]

I've going to go ahead and at least remove .makefiles from clean, this has been pretty useful for a while with that in there.

[jmalloc]

Spoke to soon, easier said than done, etc - https://github.com/make-files/lib-core/blame/v1/include/common.mk#L99

[jmalloc]

I guess we just add .makefiles to CLEAN_EXCLUSIONS by default?

[ezzatron]

Could work well in combo with a self-update target.