Add docker-style file variable support (fixes #189)

[SmileyChris]

Docker (swarm) and Kubernetes are two widely used platforms that store their Secrets in tmpfs inside containers as individual files.

These are a secure way to be able to share configuration data between containers, and having a way to access these in django-environ is useful.

Here's a quote from the official Postgres docker hub image:

As an alternative to passing sensitive information via environment variables, _FILE may be appended to some of the previously listed environment variables, causing the initialization script to load the values for those variables from files present in the container. In particular, this can be used to load passwords from Docker secrets stored in /run/secrets/<secret_name> files. For example:

[sergeyklay]

I'll take a look ASAP

[coveralls]

Coverage increased (+1.0%) to 90.909% when pulling 9f614e2 on SmileyChris:file-aware into caf1ce2 on joke2k:main.

[SmileyChris]

Oh, we're supporting 3.5 still. Easy changes to make for that.
Windows CI fails because it can't read temp files 😒

[SmileyChris]

One design decision that should be considered: currently if you use a _FILE key on a missing file (or not enough permissions), then it will fail hard with an os exception -- the thinking being that failing loud is better than silently if you explicitly asked to use a file.

[sergeyklay]

@SmileyChris Could you please update the PR descrption by adding a bit more info about purpose and possible use cases.

[SmileyChris]

@sergeyklay updated description, pushed py3.5 fix, 100% coverage, and hopefully fixed windows ci tests too

[SmileyChris]

Lint fixed (pfft, should use black's 88 line length now :D)

[sergeyklay]

In general, everything looks quite good, except for a few nitpicking from me.

[sergeyklay]

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

[panteparak]

is there a timeline for the next release that includes this PR?

[sergeyklay]

I'll try to find time for release this weekend.