Add a proper interpolation option

[potasiak]

Adds a proper interpolation mechanism that can substitute variables in the middle of values of other variables.

The current "proxy" mechanism, where a variable is substituted when set to a name of another variable prefixed with a dollar sign (VAR_A=$VAR_B) is retained, and used by default. The interpolation mechanism has to be explicitly enabled by setting Env.interpolation to True. This is done to ensure backward compatibility with all previous versions, except for v0.11.0 and v0.11.1 that have been yanked from PyPI.

This is meant to be a properly implemented "variable expansion" mechanism that can be toggled on and off, as was intended in #468 and #419.

It does not re-introduce issues that caused #485 and #490.

In addition to the instance-wide Env.interpolation setting, each method that retrieves a value from the environment got optional parameter interpolate that can enable or disable interpolation independently from the Env.interpolation. When interpolate parameter is not passed as an argument, Env.interpolation is used. This will let users handle special cases, where they need to selectively enable or disable the new interpolation mechanism.

When a variable referenced in a value of another variable is not set, an ImproperlyConfigured exception is raised. It can be disabled (missing variables will be ignored) by setting Env.raise_on_missing to False. Could not figure out a better name for now. If you have an idea - let me know.

When a variable references itself in its value, an ImproperlyConfigured exception is raised.

Variables prefixed with \$ instead of $ are ignored.

Every interpolated value is cached (up to 128 recently used variables).

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[potasiak]

After some more thinking about this, I've started to wonder if doing the interpolation when reading env, instead of on Env.get_value() call, wouldn't be a better idea. Let me know what you think.