Separate builds should truly be separate

[jeaye]

Hey @bhauman. You had mentioned here that Figwheel is following cljsbuild, but I don't think that's the case anymore. Nowadays, it seems cljsbuild handles this in stride, but Figwheel requires :load-all-builds false to be specified. Here's a minimal test case showing the behavior of both: https://github.com/jeaye/figwheel-build-issue

This took a handful of hours to realize, given that I was in a large project with several build and profile combinations and had just reorganized a lot of things. Turns out, it was just Figwheel needing a flag!

Will you please make this option default? I don't see how this would negatively affect anyone and it would follow what cljsbuild is doing.

Thanks!