Added renderWeek prop to render a custom element for week box

[jenshandersson]

I needed a way to render a custom element for week boxes on the left.
Following the renderDay implementation I created a renderWeek prop.
It defaults to the current behavior, rendering the weekNumber.

[codecov]

Codecov Report

Merging #497 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #497   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          15     15           
  Lines         534    535    +1     
  Branches      111    111           
=====================================
+ Hits          534    535    +1

Impacted Files	Coverage Δ
src/Month.js	100% <ø> (ø)	⬆️
src/DayPicker.js	100% <100%> (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 bd14401...0d9b76c. Read the comment docs.

[gpbl]

Nice @jenshandersson!

Could you please include in the PR only the changes to /src? Docs and built files don't need to be added to the PR: I'll take care of them by myself. Thanks again 👍

[jenshandersson]

@gpbl I have done what you requested, only committed the source files. Please let me know if you need anything else.

[gpbl]

Awesome thanks!!

[jenshandersson]

Thanks for merging in @gpbl! Will you have time to rebuild and update the patch version?

[gpbl]

@jenshandersson I’m going to cut a minor release in the next days 👍🏽 thanks for your patience!