fix(input): missing selector for mdPrefix

Fixes uses of `md-prefix` and `md-suffix` not being projected to the proper place in the DOM.

Fixes #5643.

Author: crisbeto

src/lib/input/input-container.html

@@ -1,7 +1,7 @@
 <div class="mat-input-wrapper">
   <div class="mat-input-flex">
     <div class="mat-input-prefix" *ngIf="_prefixChildren.length">
-      <ng-content select="[mdPrefix], [matPrefix]"></ng-content>
+      <ng-content select="[mdPrefix], [matPrefix], [md-prefix]"></ng-content>
     </div>
 
     <div class="mat-input-infix">
@@ -23,7 +23,7 @@
     </div>
 
     <div class="mat-input-suffix" *ngIf="_suffixChildren.length">
-      <ng-content select="[mdSuffix], [matSuffix]"></ng-content>
+      <ng-content select="[mdSuffix], [matSuffix], [md-suffix]"></ng-content>
     </div>
   </div>