Add the ability to specify the buildpacks file to use based on environment

[ianyh]

No description provided.

[ddollar]

What is the use case for this? This seems to add a bit of complexity for not much general gain and may be better served staying in a buildpack fork.

[ianyh]

I implemented it as a way to have two multipack stacks in the same repo. Having both stacks be in the same .buildpacks file would hit the 15 minute timeout on slug compilation.

I don't have particularly strong feelings about it being merged upstream, but I do think it's a general improvement to the flexibility of the buildpack.

[ddollar]

Hello. I apologize for not responding earlier but I am currently declaring Github bankruptcy. If this pull request or issue is still valid please feel free to re-open and explain.