Upgrade to libvips v8.6.0

Expose offset coordinates of strategy-based crop
Switch to Github releases for prebuilt libvips
Move packaging scripts to lovell/sharp-libvips repo

lib/output.js

@@ -16,6 +16,7 @@ const sharp = require('../build/Release/sharp.node');
  * @param {Function} [callback] - called on completion with two arguments `(err, info)`.
  * `info` contains the output image `format`, `size` (bytes), `width`, `height`,
  * `channels` and `premultiplied` (indicating if premultiplication was used).
+ * When using a crop strategy also contains `cropOffsetLeft` and `cropOffsetTop`.
  * @returns {Promise<Object>} - when no callback is provided
  * @throws {Error} Invalid parameters
  */