Add prebuilt variants for electron and alpine (musl)

[JCMais]

• Alpine
  See Support of Alpine Linux / Musl libc builds mapbox/node-pre-gyp#249 (comment)
• Electron
  See https://github.com/galkahana/HummusJS/blob/master/.travis.yml#L22
  https://github.com/mapbox/node-sqlite3/blob/master/.travis.yml

There is also https://github.com/prebuild/prebuild

Those are also interesting, because they remove the need to download the prebuilt files from somewhere: https://github.com/mafintosh/prebuildify https://github.com/mafintosh/node-gyp-build https://github.com/mafintosh/prebuildify-ci, those would only be interesting if using n-api (see #155)

Probably a good time to change the build scripts, and make pre built binaries have static version of libcurl. Currently this is the case only on Windows. This would fix many issues while using this lib.

[JCMais]

done for v2, currently available on node-libcurl@next