Multi Options

[Type1J]

This is really a 2 part feature request:

1. Expose raw CURLM* with a getter (...with a warning. The easy pointer already has a getter with a warning.).
2. Add multi interface options.

I'm guessing that the multi interface options are rarely changed, so there hasn't been a need for them. I have a need for them, but I'd be satisfied if just 1 (above) was done. I'm sure that others have just modified their copies of the curlpp multi header to add the getter or just make the pointer public, but they didn't file an issue. Another way this could be accessible is to make the members protected instead of private (probably what some people did, as well). Then, a user could derive a class from the curlpp multi class to get to the raw curl parts. I really like the abstractions that curlpp adds, and I don't want to have to modify the headers on all of the machines or keep my own (potentially outdatable) headers in my project, so please at least do 1 if not both 1 and 2.