Can't use method extraVars() with file path

[emek-lu]

The documentation for the method extraVars() states that extra vars loaded from files is supported, however when I run something similar to the example I get the following exception "The extra vars raw string should be in the "key=value" form."

[RoussKS]

@emek-lu I understand this is an old issue. This is still valid for v3 as it seems but not for v4

@maschmann is this something that can be sorted on v4 or was this dropped from being supported for other reasons? ie security.

might need removing from README.md

[maschmann]

@RoussKS that's a good question: Could be a regression. I've to investigate that and get back to you with an answer later. Might take a while though, since I'm pretty much occupied with a s**tload of projects at the moment and this seems to be not too pressing.

[emek-lu]

The code in #79 solves this issue. Could it be merged?