feat: Add option: 'required' by default

[bhallionOhbibi]

Changes

• Add a new option RequiredIfNoDef that mark all field ,required by default.

Why

In most of my projects, environement variables are either required or have default values.
Adding this option allow to remove a redundant (and sometimes obvious) ,required in field tags.

[caarlos0]

lgtm, but can you add a test for it as well?

[codecov]

Codecov Report

Merging #187 (be171b4) into master (929e30e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #187   +/-   ##
=======================================
  Coverage   98.13%   98.14%           
=======================================
  Files           2        2           
  Lines         322      323    +1     
=======================================
+ Hits          316      317    +1     
  Misses          4        4           
  Partials        2        2           

Impacted Files	Coverage Δ
env.go	98.10% <100.00%> (+<0.01%)	⬆️

---

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 929e30e...be171b4. Read the comment docs.

[bhallionOhbibi]

Hi @caarlos0, thanks for your feedback.
I just added a test for this new option. (be171b4)

[caarlos0]

Thanks!