Add ENV variables to customize buildpack

[inodb]

Add several new environment variables to customize buildpack:

• Add environment variables from ENV_DIR
• Add CONDA_REQ_FILE to allow using other filename then conda-requirements.txt
• Add CONDA_CHANNELS add space separated channel names to use different conda channels
• Also install using setup.py if available instead of just requirements.txt and conda-requirements.txt. Use pip install -e.

For example see: https://github.com/inodb/dicom-flask-uploader

[kennethreitz]

this looks good, but let's remove the CONDA_REQ_FILE part. Good to standardize :)