Add variable expansion

[invasy]

• Expand variables referenced as $VAR or ${VAR}.
• Detect infinite recursion in expansion (self-reference).
• Closes Any plan to support embedded variables? #421.

[coveralls]

Coverage: 92.215% (+0.3%) from 91.921% when pulling bee44b9 on invasy:feature/expansion into 69b4bc9 on joke2k:develop.

[sergeyklay]

Thank you for the patch, and for helping make django-environ better!

[elmcrest]

Hey @sergeyklay, is there already a new release available including this change?
on pypi.org I can see 0.10.0 from 3rd of march, so this can't be included. are there plans for a new release?

thx and cheers,
Marius

[sergeyklay]

Hi, @elmcrest. I'll release new version ASAP.