First and Last days extensions added

[FMorschel]

Fixes #50.

[jogboms]

I approve of this @FMorschel but do you plan to add some test cases for these?

[jogboms]

For the formatting, you can set your ide line width for this project to 120. @FMorschel

[FMorschel]

I approve of this @FMorschel but do you plan to add some test cases for these?

Tests created @jogboms

[jogboms]

[codecov]

Codecov Report

Merging #51 (23ec82b) into master (d11e087) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #51      +/-   ##
==========================================
+ Coverage   99.74%   99.76%   +0.02%     
==========================================
  Files           2        2              
  Lines         392      434      +42     
==========================================
+ Hits          391      433      +42     
  Misses          1        1              

Impacted Files	Coverage Δ
lib/src/extensions.dart	98.83% <100.00%> (+0.11%)	⬆️
test/time_test.dart	100.00% <100.00%> (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 d11e087...23ec82b. Read the comment docs.