make nav arrows configurable

[Mythra]

fixes #112

This PR was originally #154 , but I ended up making a mess of git (multiple reorders + squashing after reorder is hard 😞 ). So go look there for comments. However here is the original PR message:

This PR adds in the ability to specify navPrev, and navNext to DayPicker incase they want to override the default SVGs that come bundled with react-dates (e.g. ChevronUp, ChevronDown, LeftArrow, and RightArrow).

If navPrev, or navNext aren't specified then DayPicker will fall back to the current behavior of:

isVertical ? <ChevronUp /> : <LeftArrow />;

for the previous button, and:

isVertical ? <ChevronDown /> : <RightArrow />;

for the next button.

I also tried to make this work as much with existing code as possible. As such even with custom icons the class names: DayPicker__nav--prev, and DayPicker__nav--next are still rendered even with custom icons. If you really want to target something inside of your custom icon, you can always tag class names/ids onto the props you pass in. E.g.:

<DayPicker
  navPrev={
    <span className={'MySuperAwesomeIcon'}>
       'Prev'
    </span>
  }
/>

[Mythra]

@majapw Here's the new PR sorry about the mess I made with #154 😞

[majapw]

No problem at all! ✌️ Thank you so much for all the hard work. :) This is super. I'll merge it in as soon as tests pass and do a release today. :)

[Mythra]

🙇‍♂️ Sounds awesome! Thanks for providing such quick reviews!

[coveralls]

Coverage increased (+0.4%) to 88.726% when pulling 135f61e on SecurityInsanity:c_navigation into 213f268 on airbnb:master.

[coveralls]

Coverage increased (+0.4%) to 88.726% when pulling 135f61e on SecurityInsanity:c_navigation into 213f268 on airbnb:master.