Add url support

[padawin]

First off, thanks for this package!

I wanted to use it in our project, but we have in our environment a few URLs (DB connection strings in a single value, or URL for an image CDN for example), which should be valid ones.
It felt to me a fairly valid case to have as supported type, so here's a PR for it!

[codecov]

Codecov Report

Merging #74 (742f90d) into master (fb86a33) will increase coverage by 0.15%.
The diff coverage is 90.90%.

❗ Current head 742f90d differs from pull request most recent head 79612be. Consider uploading reports for the commit 79612be to get more accurate results

@@            Coverage Diff             @@
##           master      #74      +/-   ##
==========================================
+ Coverage   90.07%   90.23%   +0.15%     
==========================================
  Files           1        1              
  Lines         252      256       +4     
==========================================
+ Hits          227      231       +4     
  Misses         17       17              
  Partials        8        8              

Impacted Files	Coverage Δ
cleanenv.go	90.23% <90.90%> (+0.15%)	⬆️

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[ilyakaznacheev]

Thanks for the PR! It looks like a really useful feature.

Please fix a couple of details before the merge.

[padawin]

I'm sorry, my github notifications fell through the cracks apparently, and I completely forgot about this PR and did not see the comments.

I will tackle them ASAP!

[padawin]

Here they are, let me know if you are happy with them!

[ilyakaznacheev]

Well, it took me some time

[ilyakaznacheev]

Thanks for your contribution!