Feeding dotenv via stdin: use "-" as the data file

Version bump: 0.3.11

closes kolypto#40: env() function ; documentation fix

closes kolypto#38: Customization: J2, context, etc

Jinja2: enabled extensions

filters: |env lets you use environment variables in any template

fixes kolypto#33: PyYAML CVE fixed, using the safe FullLoader

See https://github.com/yaml/pyyaml/wiki/PyYAML-yaml.load(input)-Deprecation

closes kolypto#31: --undefined option to support the use of undefined…

… variables

closes kolypto#30: -o outfile

Release: 0.3.3