First pass at official user-env-compile plugin

- Uses ENV and APP/ENV files
- Supports old BUILD_ENV files (which are likely in wide-use)
- Allows user's to override globals with app-specific configuration

Closes #284
Closes #683
Closes #763

Refs #503
Refs #678
Fix reference to $APP
create build env for both dokku and new style buildpacks
link to commit id and line in comment
Merge pull request #812 from expa/build-env

create build env for both dokku and new style buildpacks
build-env todos
Merge pull request #813 from expa/build-env

build-env todos
Merge pull request #787 from progrium/build-env

Official user-env-compile plugin