v0.20.0

• feat: add predefine options: EMNAPI_WORKER_POOL_SIZE, EMNAPI_NEXTTICK_TYPE, EMNAPI_USE_PROXYING
• refactor: add uv.h and port libuv's threadpool to schedule async works.
• fix: #8

Breaking Changes

If you use multithreading API such as napi_*_async_work or thread safe functions, you need to build additional source files (src/uv/**/*.c), so you need to add them to your makefile or build script, or directly use CMake add_subdirectory() and target_link_libraries().