Add gravity support to embed feature (#1038)

lib/resize.js

@@ -179,11 +179,26 @@ function crop (crop) {
  *     // outputBuffer contains WebP image data of a 200 pixels wide and 300 pixels high
  *     // containing a scaled version, embedded on a transparent canvas, of input.gif
  *   });
- *
+ * @param {String} [embed='centre'] - A member of `sharp.gravity` to embed to an edge/corner.
  * @returns {Sharp}
+ * @throws {Error} Invalid parameters
  */
-function embed () {
+function embed (embed) {
   this.options.canvas = 'embed';
+
+  if (!is.defined(embed)) {
+    // Default
+    this.options.embed = gravity.center;
+  } else if (is.integer(embed) && is.inRange(embed, 0, 8)) {
+    // Gravity (numeric)
+    this.options.embed = embed;
+  } else if (is.string(embed) && is.integer(gravity[embed])) {
+    // Gravity (string)
+    this.options.embed = gravity[embed];
+  } else {
+    throw is.invalidParameterError('embed', 'valid embed id/name', embed);
+  }
+
   return this;
 }