added validuntil option

[slinso]

I needed some kind of TTL for the URL, so it is possible to expire public URLs to an image, so passing image URLs around and requesting the image is not possible an infinite time.
My simple approach was to add a validUntil Parameter to the Options

vu{date:20060102}

I added the option to the signature so it is not possible to change the validity of the URL.
For my use-case it is ok if images are only valid a single day. otherwise you would have to put some thought into URL generation in your own app to always use the same date, so browser caching would be possible longer than a single day.

What do you think?

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[slinso]

@googlebot I signed it!

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[codecov]

Codecov Report

Merging #222 into master will decrease coverage by 1.43%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #222      +/-   ##
==========================================
- Coverage   89.16%   87.73%   -1.44%     
==========================================
  Files           6        6              
  Lines         674      685      +11     
==========================================
  Hits          601      601              
- Misses         50       59       +9     
- Partials       23       25       +2     

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[AnTheMaker]

Hi there! This sounds like a really useful addition! Any updates on this?