add daterange component

[RT567]

No description provided.

[Gregg8]

My first comment is that it's really nice and feature complete. Having said that, I do have the following comments on the UI:

• The styling of this component is not similar enough to the other re-com components, in particular the datepicker. It's certainly more contemporary and I do really like it, however we should bring it back to a similar look and feel to the others. Additionally, I have a couple of sub-points
  • I see throughout the UI (and the code) currently Monday and Wednesday are represented with single letters, so they should be two letters to match the other days (or even possibly three letters (with small-caps formatting) to match the current datepicker)
  • Another inconsistency is that daterange doesn't fill the entire grid with the dates of the days in previous and next month. I'm not saying this is bad, but datepicker does do that, and I do actually find it handy sometimes, so please fill in those empty cells
• :disabled? doesn't prevent navigation. I know it's in the docs but I see that datepicker also disables navigation, so I would think we should make them both consistent. Happy to discussion, given that perhaps the reason navigation is not disabled is because the selected date range might span more than two months?
• When you select a date range but then single click on a date, which deselects the selected date range, the external model does not match the state of the component. I see that this is documented but I am questioning (suggesting) model be updated to nil. If a user wanted to cancel the date range, they would leave it in this incomplete state, however the value of the component to the calling code would be the last valid date range
• The dropdown version drops down right justified but if there's no room underneath, it drops up center justified. I do note that the datepicker does exactly the same thing, so I wouldn't worry about this one
• Very optional stretch goal: Could allow click-n-drag in addition to click-n-click (more marks for auto-next/previous month when dragging beyond left/right)

Also, I have now merged this PR into re-com proper as a branch (https://github.com/day8/re-com/tree/RT567-master) and added a commit of my own which contains coding comments and changes.

Please review that commit along with the above then we can have a session to discuss your thoughts when you are ready

Any changes you now make should be done in this branch, not you PR, but we should probably have our chat before you make further changes.

Nice work!