N-API Improvements

[jschlight]

This pull request implements a number of improvements to the node-pre-gyp N-API support for issue #402.

N-API builds for only those N-API versions supported by the Node instance

node-pre-gyp now verifies the N-API build versions being requested and builds only those versions supported by current Node instance. If no version can be built, an error message is generated.

New --build-latest-napi-version-only build argument

A new --build-latest-napi-version-only build argument instructs node-pre-gyp to build only the highest N-API version supported by the Node instance. This is useful in CI and test environments where only the most current N-API build is needed.

Support for NAN and N-API builds from the same project

This PR introduces the ability to build both N-API and NAN binaries from the same package.

Placing the configuration value {node_napi_label} in the package.json binary.package_name property instructs node-pre-gyp to build all viable N-API binaries supported by the current Node instance. If the current Node instance does not support N-API, node-pre-gyp will request a traditional, non-N-API build.

The configuration value {node_napi_label} is set by node-pre-gyp to the type of build created. For N-API builds, the string contains the N-API version with values like napi-v3. For traditional, non-N-API builds, the string contains the ABI version with values like node-v46.

The C/C++ symbol NAPI_VERSION can be used to distinguish N-API and non-N-API builds.

There are more details in the updated main README.md file.

Build support for Node 4

This PR fixes a JavaScript language issue that was preventing builds against some versions of Node 4.

Better recovery from build errors

A configure is no longer required in the case where a build fails.

NAPI_VERSION replaces NAPI_BUILD_VERSION

Previously, the documentation recommended defining the C/C++ symbol NAPI_BUILD_VERSION. The documentation now recommends the symbol NAPI_VERSION which is used by the N-API C/C++ headers to define the API for that specific N-API version.

[springmeyer]

Thank you @jschlight