Add navbarElement and weekdayElement props

[boatkorachal]

from #177

[gpbl]

Hi @boatkorachal thanks for the help and congrats for your first PR 🎊

To maintain backward compatibility, we should not remove these *Component props. My idea was actually to add two new props. We could eventually deprecate them. Sorry for not being clear enough: I'll add some comments to your PR.

[gpbl]

(You should rebase your branch with master to fix these conflicts)

[gpbl]

Keep those lat two lines. I'll add a PropType to deprecate them. Same for the other changes :)

[boatkorachal]

ewww i just did the rebase and force push. hope that i haven't done anything wrong :)

[coveralls]

Coverage decreased (-0.3%) to 99.732% when pulling 34bff97 on boatkorachal:navbar-and-weekday-element into 405d62a on gpbl:master.

[gpbl]

Yay thanks! Could you even test that the rendered component is actually the navbarElement? 🙏
https://coveralls.io/builds/6541993/source?filename=src%2FDayPicker.js#L357

[boatkorachal]

Isn't it checked at proptypes? or should I import react-addons-test-utils and change to something like this

if ( TestUtils.isElement(navbarElement) ) {
  return React.cloneElement(navbarElement, props)
}

[gpbl]

The test coverage has been decreased, this is because we didn't include a test to check if passing a navbarElement did actually render the element. See https://coveralls.io/builds/6541993/source?filename=src%2FDayPicker.js#L357

Actually, even weekdayElement hasn't be checked: I wonder we are actually rendering it :)
I can't work on this now, so that's why I'm asking :)

[boatkorachal]

I have just update tests. Is it ok? (I have to admit that this is the first time i write the tests too haha)

[boatkorachal]

oh sorry i forgot to run lint. i'm gonna fix it now.

[coveralls]

Coverage remained the same at 100.0% when pulling 9878237 on boatkorachal:navbar-and-weekday-element into 405d62a on gpbl:master.

[gpbl]

Looks good! Thanks a lot!