Fix #1967 Add auto mode for start/end of week

[kylekatarnls]

Fix #1967

Using:

<?php

Carbon::setWeekStartsAt('auto');
Carbon::setWeekEndsAt('auto');

Carbon::setLocale('en_US'); // or en which is also the default locale

echo Carbon::now()->startOfWeek()->dayName; // Sunday
echo Carbon::now()->endOfWeek()->dayName; // Saturday

[codecov-commenter]

Codecov Report

Merging #2090 into master will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##              master     #2090   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
- Complexity      1955      1960    +5     
===========================================
  Files            882       882           
  Lines           8520      8528    +8     
===========================================
+ Hits            8520      8528    +8     

Impacted Files	Coverage Δ	Complexity Δ
src/Carbon/Traits/Options.php	100.00% <ø> (ø)	22.00 <0.00> (ø)
src/Carbon/Traits/Date.php	100.00% <100.00%> (ø)	282.00 <1.00> (+5.00)

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