Add a Pop button

[dcipher00]

Add a Pop button in the centre of the bottom Stack navigator so we can use it to flash a card in the future implementation of word for the day.

[Alexandria]

Hello! I'd like to take a crack at this.

[dcipher00]

@Alexandria sure, go ahead and feel free to raise a PR

[Alexandria]

@dcipher00 Do you want this button to point to a blank screen for now?

[dcipher00]

@Alexandria yes please create a blank screen for now. The basic idea behind the pop button is that we have to flash a card on top rather than a complete screen navigation so you don't need to create a screen. just create a card with a cross button which would appear once pop button is clicked.