Add support for formatDate to DateTimePicker.

Based on pnp#240 by @AJIXuMuK

docs/documentation/docs/controls/DateTimePicker.md

@@ -50,7 +50,7 @@ The `DateTimePicker` control can be configured with the following properties:
 | ---- | ---- | ---- | ---- |
 | label | string | no | Property field label displayed on top. |
 | disabled | boolean | no | Specifies if the control is disabled or not. |
-| formatDate | function | no | Defines a formatDate function to display the date of the custom Field. By defaut date.toDateString() is used. |
+| formatDate | function | no | Defines a formatDate function that can override the output value in Date picker. |
 | dateConvention | DateConvention | no | Defines the date convention to use. The default is date and time.|
 | timeConvention | TimeConvention | no | Defines the time convention to use. The default value is the 24-hour clock convention. |
 | firstDayOfWeek | DayOfWeek | no | Specify the first day of the week for your locale. |

src/controls/dateTimePicker/DateTimePicker.tsx

@@ -305,6 +305,7 @@ export class DateTimePicker extends React.Component<IDateTimePickerProps, IDateT
       showMonthPickerAsOverlay = false,
       showWeekNumbers = false,
       showSeconds = false,
+      formatDate,
       value = this.state.day,
       strings: dateStrings = new DateTimePickerStrings() // Defines the DatePicker control labels
     } = this.props;
@@ -369,6 +370,7 @@ export class DateTimePicker extends React.Component<IDateTimePickerProps, IDateT
             </div>
             <div className={styles.picker}>
               <DatePicker
+                formatDate={formatDate}
                 disabled={disabled}
                 value={value}
                 strings={dateStrings}

src/webparts/controlsTest/components/ControlsTest.tsx

@@ -363,6 +363,7 @@ export default class ControlsTest extends React.Component<IControlsTestProps, IC
 
         <DateTimePicker
           label="DateTime Picker (Controlled)"
+          formatDate={d => `${d.getFullYear()} - ${d.getMonth() + 1} - ${d.getDate()}`}
           dateConvention={DateConvention.DateTime}
           timeConvention={TimeConvention.Hours24}
           firstDayOfWeek={DayOfWeek.Monday}