Handle escaped dollar sign in values

[mehdy]

Fixes #271

[coveralls]

Coverage increased (+0.07%) to 90.998% when pulling 1b6c4ba on mehdy:develop into 7dc1c1b on joke2k:develop.

[mehdy]

@sergeyklay ready for review.

[sergeyklay]

@mehdy Thank you for the patch.

In fact, I'd like to go this way:

1. Make current interpolation feature optional and provide an ability to disable it v0.8.0
2. Implement a brand new interpolation feature using the following form: FOO=${BAR} v0.9.0
3. Disable by default old feature v0.9.0
4. Enable by default new feature v1.0.0
5. Drop old feature v1.0.0

I'm trying to understand how much your patch might hinder or help with this plan.

[mehdy]

@sergeyklay You're welcome!

I think I can add a feature flag to the escaping logic so it meets the first step's requirements.
Should I go for it?

[sergeyklay]

Yeah, it would be better. Also could you disable it by default?

[mehdy]

@sergeyklay What do you think of this?

[sergeyklay]

@mehdy Looks as I expected, thanks! Could you please add a quick note to the documentation? Feel free to edit the tips.rst to show possible use cases and examples.

[mehdy]

@sergeyklay Sure!

[mehdy]

@sergeyklay I think we're good to go!

[sergeyklay]

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