Allow user to exclude packages from .py trimming

Context

Zappa excludes .py files as it packages up Python directories if a .pyc/.pyo exists. However some tools need to find the .py file, in my case Pyramid's plugin extension locator.

Expected Behavior

The zipped package should have .py files if needed as some Python packages expect to find them.

Actual Behavior

All .py files are removed.

Possible Fix

Provide a zappa settings option to specify packages that should be excluded from this process.

Steps to Reproduce

1. Package up anything that has .pyc files
2. Look at the resulting zip that has no .py files.