Include placeholder mixin in input (#1257)

...so that its color is consistent with $input-color

sass/elements/form.sass

@@ -45,6 +45,8 @@ $help-size: $size-small !default
   background-color: $input-background-color
   border-color: $input-border-color
   color: $input-color
+  +placeholder
+    color: rgba($input-color, 0.3)
   &:hover,
   &.is-hovered
     border-color: $input-hover-border-color