Add, document form field full-width modifier

Signed-off-by: Alex Coles <alex@alexbcoles.com>

app/assets/stylesheets/content/_forms.md

@@ -99,6 +99,16 @@
         This field has no label, which means you really can write what you like.
       </div>
     </div>
+    <div class="form--field -required -full-width">
+      <div class="form--field-container">
+        <div class="form--text-field-container">
+          <input type="text" class="form--text-field">
+        </div>
+      </div>
+      <div class="form--field-instructions">
+        This field also has no label, but takes up the full width.
+      </div>
+    </div>
   </section>
 </form>
 ```

app/assets/stylesheets/content/_forms.sass

@@ -257,6 +257,9 @@ fieldset.form--fieldset
   .form--field.-no-label &
     @include grid-offset(2)
 
+  .form--field.-full-width &
+    @include grid-size(12)
+
   .form.-wide-labels &,
   .form--field.-wide-label &
     @include grid-size(8)
@@ -277,6 +280,10 @@ fieldset.form--fieldset
   line-height:  $base-line-height
   color:        $form-label-color
 
+  .form--field.-full-width &
+    @include grid-size(12)
+    @include grid-offset(0)
+
   .form.-wide-labels &,
   .form--field.-wide-label &
     @include grid-size(8)