Add support for option annotation/comments

[troglobit]

It would be neat if we could add option annotation support to libConfuse:

/* Some comment to an option */
option = value

/* Section comment, here be dragons */
section {
    key = val
    /* Another comment, why we set to true */
    bool = true
}

So that if we call cfg_print() on the above parsed cfg_t, we would get the same output.

The following variant should also be possible:

option = value              /* Some comment to an option */

but with the following the last comment would be set

/* Some comment to an option */
option = value              /* Actual comment set to to option */

[troglobit]

Skipping second variant (post-comment), due to complexity to implement, for v3.1