Default a Field's placeholder to the label

src/components/views/elements/Field.js

@@ -25,7 +25,7 @@ export default class Field extends React.PureComponent {
         type: PropTypes.string,
         // The field's label string.
         label: PropTypes.string,
-        // The field's placeholder string.
+        // The field's placeholder string. Defaults to the label.
         placeholder: PropTypes.string,
         // The type of field to create. Defaults to "input". Should be "input" or "select".
         // To define options for a select, use <Field><option ... /></Field>
@@ -55,6 +55,7 @@ export default class Field extends React.PureComponent {
         // Set some defaults for the element
         extraProps.type = extraProps.type || "text";
         extraProps.ref = "fieldInput";
+        extraProps.placeholder = extraProps.placeholder || extraProps.label;
 
         const element = this.props.element || "input";
         const fieldInput = React.createElement(element, extraProps, this.props.children);