Add a get-or-insert method

[svenstaro]

Hey, super cool library. I really like the idea!

I'm missing an atomic get-or-insert method, though. I haven't look at this library's internals yet but I think it could work like:

1. Lock the shmap
2. Get value if exists
3. Insert using provided function/value otherwise
4. Unlock the shmap

I think it should be doable.

Currently I'm implementing this using an external lock around the shmap (which kind of defeats the purpose).