adding support for custom env variables
Update README.rst

fixing readme
Update README.rst

fixed readme
Merge branch 'master' into feature/custom-env-vars
Fix trailing whitespace
Test missing custom variable behaviour

With current implementation, this test fails because missing environment
variables aren't set.

I don't think this is ideal behaviour (I think if you say `d12f("FOO")`
it's reasonable to expect the presence of `settings.FOO`) - I want to
think about it more before it goes into mainline. I can see the sense
behind both.
Set unset custom env vars to None
Use named parameter in example code
Merge branch 'master' into feature/custom-env-vars
Added CHANGELOG entry
Merge branch 'feature/custom-env-vars'

Grab arbitrary environment variables as settings with the `custom_settings` parameter!
1.1 - Add `custom_settings`