Gallery : Change the snap transition duration and easing with props

[mcoeur]

I needed to adjust the duration of the transition that occurs when an image snaps to the side of the screen.
My approach was to set those options with props (with a fallback to default options).

Example use :

<Gallery
  data={assets}
  renderItem={renderItem}
  snapTransitionConfig={{ duration: 180, easing: Easing.ease }}
/>

I adjusted the documentation accordingly.

Any thoughts ?