[v3] Add ability to apply transition to bottom screen

[christianchown]

Hi Sergey - looking for another bit of v2 functionality to be added to v3, something like #30.

I'm thinking of an API like

<Navigator animations={{
  fadeRight: {
      start: { opacity: 0, transform: [{ translateX: windowWidth }] },
      end: { opacity: 1, transform: [{ translateX: 0 }] },
      push: { opacity: 0, transform: [{ translateX: -windowWidth / 3 }] },
      pop: { opacity: 1, transform: [{ translateX: 0 }] },
  }
}}/>

Where push and pop are optional, but get applie:

• (push) to stack[stack.length-1] as the end position when a push/replace/reset happens, and to
• (pop) to stack[stack.length-2] as the end position when a pop happens

Can't think of good names for them though... (bottomPush/bottomPop ? topPush/topPop?)