Multi addon projects

[kraenhansen]

This draft PR adds an ability for cmake-rn to build projects declaring multiple addons in a single invocation.

Currently drafted, since I need to figure out how to handle the final output.

The current state of this PR produce prebuilds located in packages/node-addon-examples/build instead of a ./build directory next to the individual source folders:

> ls RelWithDebInfo async-test-addon.android.node example-1-hello.apple.node example-4-addon.android.node example-7-addon.apple.node
async-test-addon.apple.node example-10-binding.android.node example-4-addon.apple.node example-8-addon.android.node
buffers-test-addon.android.node example-10-binding.apple.node example-5-addon.android.node example-8-addon.apple.node
buffers-test-addon.apple.node example-2-hello.android.node example-5-addon.apple.node example-9-addon.android.node
example-0-hello.android.node example-2-hello.apple.node example-6-addon.android.node example-9-addon.apple.node
example-0-hello.apple.node example-3-addon.android.node example-6-addon.apple.node
example-1-hello.android.node example-3-addon.apple.node example-7-addon.android.node