add new .img-fluid class

src/scss/core/_images.scss

@@ -0,0 +1,8 @@
+// A class, .img-fluid, shall be used to create responsive images. 
+// Note: having <img> responsive by default can lead to breakage.
+// .img-fluid gives designers more control over sizing.
+// Also, third party widget/plugins are not overriden by default.
+
+.img-fluid {
+  @include img-fluid();
+}