Add sum-if sample demonstrating lambdas.

[nigham]

No description provided.

[sftrabbit]

Thanks. I'll be catching up on updating the site this coming weekend, so I'll take a look then.

[sftrabbit]

I feel like a better sample would use std::accumulate with a conditional predicate. However, perhaps even better would just be a simple sample demonstrating how to use std::accumulate! Will come back to this later.

[sftrabbit]

I've created issue #62 for creating a simpler std::accumulate sample.