Add global expression "date"

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

src/meta.ts

@@ -323,6 +323,7 @@ export class Meta {
 
   private setGlobalExp(val): string {
     const ctx = this.context;
+    const currentDate = this.date;
     return handlebars.compile(val)({
       branch: function () {
         if (!/^refs\/heads\//.test(ctx.ref)) {
@@ -338,6 +339,9 @@ export class Meta {
       },
       sha: function () {
         return ctx.sha.substr(0, 7);
+      },
+      date: function (format) {
+        return moment(currentDate).utc().format(format);
       }
     });
   }