feat(MdTable): allow 2D array as the model value (vuematerial#1477)

Author: azzzy

src/components/MdTable/MdTableRow.vue

@@ -28,7 +28,7 @@
       },
       mdDisabled: Boolean,
       mdAutoSelect: Boolean,
-      mdItem: Object
+      mdItem: [Array, Object]
     },
     inject: ['MdTable'],
     data: () => ({

src/components/MdTable/MdTableRowGhost.vue

@@ -5,7 +5,7 @@
     props: {
       mdIndex: [String, Number],
       mdId: [String, Number],
-      mdItem: Object
+      mdItem: [Array, Object]
     },
     render () {
       this.$slots.default[0].componentOptions.propsData.mdIndex = this.mdIndex