Support cmake-js

[thegecko]

Fixes #49

This PR adds support for driving cmake-js in the same way as prebuild by using the --backend flag. e.g.:

prebuildify --backend cmake-js -- --prefer-clang

[vweevers]

I prefer the generic approach of #7.

[thegecko]

I prefer the generic approach of #7.

AFAICT that removes all build support and instead the user needs to define their own external script command?

That approach is quite a breaking change as all users of prebuildify would need to all specify (the same) replacement build script externally. This feels opaque and not very user friendly.

If there were a lot of common build backends, I can see the benefit of that approach, but I'm not aware of anything outside of just node-gyp and cmake-js.

Another benefit of this approach is it aligns with the existing prebuild package.