Summary:
A library that provides gflags-like functionality, with the addition of:
- Threadsafe read/writes for all flags
- Arbitrary C++ types
- No declare/define split, all setting definitions are declaration and are safe to place in headers
- Runtime introspection of all flags
- Compile time and runtime collision checks
- Namespaces per project

Down the road:
- Overlay support (change a set of settings based on some context)
- Construct arbitrary user types from string
- Support multiple library instances with distinct settings in a single process
- Callbacks on setting updates

Reviewed By: aary, mnv104

Differential Revision: D6217956

fbshipit-source-id: 7c03d79c7601ce7b266df78e62b7193712d8331e