The--import-env/-e option is broken

[Sean Wellington]

Setting a value for the --import-env/-e option causes the program to crash with the following error:

Traceback (most recent call last):
File "/Users/sean/Source/gcp-proto/venv/bin/j2", line 10, in
sys.exit(main())
File "/Users/sean/Source/gcp-proto/venv/lib/python3.7/site-packages/j2cli/cli.py", line 206, in main
sys.argv[1:]
File "/Users/sean/Source/gcp-proto/venv/lib/python3.7/site-packages/j2cli/cli.py", line 163, in render_command
args.import_env
File "/Users/sean/Source/gcp-proto/venv/lib/python3.7/site-packages/j2cli/context.py", line 207, in read_context_data
context[import_env] = environ
TypeError: 'filter' object does not support item assignment

This happens for all input formats.