Added a readOnly prop to the SingleDatePicker

[erin-doyle]

This will allow the DateInput to be set as readOnly (addresses #496).

[ljharb]

This LGTM, but it needs a rebase (to remove the merge commit)

[erin-doyle]

Ok, I think I've done the rebase properly. Let me know if not.

[majapw]

I think if we're going to add this to the SingleDatePicker, it makes sense to add it to the DateRangePicker too, no?

Also, I do think we'll be paying some attention to revamping the inputs in the near future so that will likely help work on the problems this prop is trying to solve.

[ljharb]

@erin-doyle no, there's still merge commits in the PR. Assuming origin is your fork and source is this repo, git checkout readonly_prop_#496 && git fetch source && git rebase source/master && git push -f origin readonly_prop_#496 should do it assuming there's no conflicts.

[coveralls]

Coverage remained the same at 86.83% when pulling c73eca3 on erin-doyle:readonly_prop_#496 into 768578e on airbnb:master.

[erin-doyle]

Ok, I've rebased (sorry about that, I am not too strong with the git-fu yet 😜 ) and added the prop to DateRangePicker.

[coveralls]

Coverage remained the same at 86.83% when pulling e36396f on erin-doyle:readonly_prop_#496 into 768578e on airbnb:master.

[majapw]

Lol pass-through props. Looks great!

[erin-doyle]

Great, thanks! 😄

[majapw]

Woo I'll do a release shortly after I get this one other PR in :)