feat: propagate process environment variables

lib/command.js

@@ -238,7 +238,7 @@ module.exports = function (proto) {
       return cb(new Error("imageMagick does not support minify, use -scale or -sample. Alternatively, use graphicsMagick"));
     }
     try {
-      proc = spawn(bin, args);
+      proc = spawn(bin, args, {env: process.env});
     } catch (e) {
       return cb(e);
     }

lib/compare.js

@@ -96,7 +96,7 @@ module.exports = exports = function (proto) {
     var cmd = bin + ' ' + args.map(utils.escape).join(' ')
     debug(cmd);
 
-    var proc = spawn(bin, args);
+    var proc = spawn(bin, args, {env: process.env});
     var stdout = '';
     var stderr = '';
     proc.stdout.on('data',function(data) { stdout+=data });