Add stringstream support.

[IruzzArcana]

As title suggests, adds an overload for INIFile which accepts a stringstream pointer, alongside two functions called readbuffer and writebuffer.

Example usage:

//reading
mINI::INIStructure ini_data;
std::stringstream inibuffer;
inibuffer << "[section]\nkey=value\nkey2=something";
mINI::INIFile file(&inibuffer);
file.readbuffer(ini_data);

//writing
mINI::INIStructure ini_data;
ini_data["section"]["key"] = "value";
std::stringstream inibuffer;
mINI::INIFile file(&inibuffer);
file.writebuffer(ini_data);

[metayeti]

Sorry this is taking a bit but I just haven't had time to look into it. It seems like it adds quite a lot of code though, I'm not sure if this functionality merits adding it. I'll look into it eventually and see whether or not this make sense to me. Thank you for the PR though!

[bramtechs]

As mentioned in #35, I would love to see this feature added! I receive files over HTTP that are then only stored in memory.
The decision is ofcourse up to the maintainer. In the meantime I'll be relying on this fork.