Make Record default move constructible

[3XX0]

This allows move-constructing Records in C++11 and above

[SergiusTheBest]

Hi!

I think Record is never copied or moved. Where do we need that?

[3XX0]

This isn't used in plog itself but this is useful for the application developer.
For example if one wants to save Records in standard containers (say a std::vector<Record>) to defer logging.

[SergiusTheBest]

Yes, it makes sense.