Add CMake option for install into moodycamel

[Chardrazle]

Hi, I tend to use your excellent queues in various placements.

I recently noticed that the concurrentqueue cmake build installs with the ".../moodycamel/" sub-directory. However, this readerwriterqueue build does not have the moodycamel part.

I tend to root the include directories to work as: #include <moodycamel/concurrentqueue.h>

As you wouldn't want to break existing code, any chance of a cmake option that would add the "moodycamel/" part into the readerwriterqueue install location?