Pyaml-env

Resolves issue 30

- [x] You can now enforce datatype of environment variable or default value with `tag:yaml.org,2002:datatype` e.g. `tag:yaml.org,2002:float` (workaround because `!!float` is not allowed when there is an `!ENV` tag. (see 26 )
- [x] Bug fix: parse only environment variables that are denoted with `!ENV` tag, not any environment variable (see 25 )

- [X] Minor change in requirements.txt to allow PyYAML 6.* too.

**Changelog:**
- [X] Added an encoding option for opening files - defaults to utf-8
- [X] Handling of `default_value=None`

Adding `loader` kwarg to `parse_config` to address 17 .

Be more flexible with PyYAML version.