Feature: Turning datepicker into a controlled component

[arthurdenner]

What kind of change does this PR introduce?
You can now use onDateChange and selected props to control the datepicker, just like you would use onChange and value in an input element.

Also, it adds a default prop to firstDayOfWeek and a z-index to the calendar component.

What is the current behavior?
The state of the datepicker is not updated when a new selected prop is provided.

What is the new behavior?
When a new selected prop is provided, the datepicker recalculates its state.

Checklist:

• Documentation
• Tests
• Ready to be merged
• Added myself to contributors table

[codecov-io]

Codecov Report

Merging #4 into master will decrease coverage by 0.16%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master      #4      +/-   ##
=========================================
- Coverage   10.12%   9.95%   -0.17%     
=========================================
  Files          13      13              
  Lines         237     241       +4     
  Branches       44      45       +1     
=========================================
  Hits           24      24              
- Misses        173     176       +3     
- Partials       40      41       +1

Impacted Files	Coverage Δ
src/components/datepicker.js	0% <0%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 014cb44...6b0fec3. Read the comment docs.