Auto type env var injector

[workanator]

Hello,
I added the auto type resolution env var injector. It tries to match the type of the underlying value using existing parsers.

Example:

export MY_VAR1=True;
export MY_VAR2=42;

some_auto_var1 = $"MY_VAR1"; // The result type is ScalarValue::Boolean(true)
some_auto_var2 = $"MY_VAR2"::auto; // The result type is ScalarValue::Integer32(42)

[filipegoncalves]

Please add at least a TODO here to handle the error case. Arguably, the library needs a refactor to generate proper errors, and I intend to do that soon, so please make life easier for me and add a note here.

Please follow this format:

// TODO(your_username) TODO-description-here

Or if you want to leave it up to me, you can use // TODO(filipegoncalves). See the example TODO line I added on your other PR where you did something similar.

[filipegoncalves]

Closing / reopening to force travis rebuild (library was updated to use nom 1.0)