Add support to node-webkit target.

[saboya]

This PR adds support to node-webkit target, necessary to build native modules when using NW.js, an Electron alternative.

Tests are included.

NW.js apparently does not follow a specific versioning pattern to switch ABI, so the getNextTarget function can't be implemented correctly for node-webkit, so I left it untouched.

[ralphtheninja]

LGTM!

[lgeiger]

@saboya Thanks for adding this :+1:

@ralphtheninja wouldn't this be a breaking change for prebuild since this PR modifies allTargets which is used to determine the default ABIs to prebuild against?

Is this behavior change in behavior desired? If so we should do a new major release, else I would recommend adding node-webkit to something like additionalTargets so we don't break earlier versions of prebuild.

[ralphtheninja]

Is this behavior change in behavior desired? If so we should do a new major release, else I would recommend adding node-webkit to something like additionalTargets so we don't break earlier versions of prebuild.

Moving to additionalTargets is probably a good idea.

[ralphtheninja]

@lgeiger Maybe should have a discussion on what is ok for default behavior of prebuild. I'll open an issue.

[ralphtheninja]

@lgeiger I've made PR to adjust this. Hold off with merge.

[ralphtheninja]

Some research below:

[lgeiger]

@saboya @ralphtheninja Thanks for your work!

This is released under 2.2.0