Add flexibility to datepicker component

[JBurkinshaw]

Make the datepicker component more flexible based.

Describe the solution you'd like

• Make the calendar button customizable (turn it into a prop and keep the Font Awesome button/icon as the default).
• Allow changing the icon's position (i.e. at the beginning or the end of the component)
• Enable placeholder text within the input when no date value is selected.

Describe alternatives you've considered

• Make only the icon within the blue button customizable (e.g. pass in a prop to specify a different icon)
• Lots of choice for either an option to put the label text within the element as a placeholder instead of passing in placeholder text as a prop.