[DatePicker] add prop: openImmediately={true}

[liesislukas]

Description

I'm making editable text inputs. There is text, click on it - opens input: text field focused in, dropdown selections etc. Can't make it happen with date picker, because there is no option to open DatePicker automatically after render. I will make input's styles to look like there's no input

<DatePicker
            textFieldStyle={{fontSize: 'inherit'}}
            underlineShow={false}
            ...
/>

but it's dirty way and input will actually be rendered, so a lot more DOM elements on view. I have big tables with many dates.

Please add option to DatePicker like we have with DropDownMenu openImmediately={true}

Images & references

<DatePicker
          openImmediately={true}
          ...
/>

[oliviertassinari]

Closing for #4787