Making it compatible with gcc 4.8

[rafadesu]

There is an inline function that assumes the compiler has copy elision
optimization which is actually not required until C++17. GCC 4.8 does it
but it requires the copy constructor to be defined which is not the case
for std::ifstream. Fixed by passing reference by parameter.

[rafadesu]

Congratulations on such a great library.
Unfortunately, we are stuck with c++4.8 at the moment for the project we would like to use it in and this is the only problem it has while compiling. I'd appreciate if you considered this change so we don't have to patch or use a fork.
By the way, what are the plans to create a new release? It looks like a very long time since the previous one was created.

[pantor]

Thanks for your PR, it looks good to me.

I‘ll probably release version 3.0 very soon, but I want to add a few more unit tests.