Read the download url of librdkafka from Windows environment variable

[yunnysunny]

In Windows ,the download of the library of librdkafka may be so slowly, especially when you are in China. It may cost a long time to download it. Sometime I have to wait for several hours.
So I add an feature of reading the download url from environment variable.

[giulliano-bueno]

LGTM.

[yunnysunny]

@iradul Consider this pull request?

[iradul]

There is this BUILD_LIBRDKAFKA env variable that can be used to compile against system installed version of librdkafka. This doesn't work on Windows but I think it would be great if we could make it work and hence have consistency across platforms.

[yunnysunny]

I read the code of binding.gyp, the BUILD_LIBRDKAFKA is only an flag to indicate whether to building the source code with the shared library installed in OS already. And my pull request aims at supplying an URL to download nuget package.
So I think it's two different thing.
Or do you mean just rename the env name DOWNLOAD_BASE_URL_LIB_RDKAFKA to BUILD_LIBRDKAFKA?

[yunnysunny]

I have renamed DOWNLOAD_BASE_URL_LIB_RDKAFKA to BUILD_LIBRDKAFKA.

[iradul]

Let's call this variable NODE_RDKAFKA_NUGET_BASE_URL and also please leave README file unchanged, I'll update it after I merge this PR.

[yunnysunny]

done

[yunnysunny]

Expecting this merge can be published to npm. We can not bear the network in China now.

[iradul]

@yunnysunny I just released the new version