Introduce BP_DIRECT_PROCESS to enable run images without shell

[c0d1ngm0nk3y]

Fixes #246

Summary

This pr introduces BP_DIRECT_PROCESS to force the processes from the Procfile to be started without shell.

Use Cases

A run image without shell with another stack other stack than the hardcoded tiny ones. See #246 for details.

Checklist

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• I have added an integration test, if necessary.
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[dmikusa]

This looks good, in addition to your technical description in the README, I would just like to position this feature as a way for users to prepare. Because we will be moving to having everything be a direct process (forced by CNB), I'd like to describe this new env variable in the README as a way for users to opt-in now and prepare for this change.

I'd also like the README to give notice now that a.) the next major release of procfile buildpack will make the change to requiring direct processes and b.) once we require everything to use direct processes this setting will be removed.