Add option to provide current time when decoding JWT

[ZipFile]

• Makes possible to fake a time without using freezegun:freeze_time or unittest.mock:patch. Less monkey patching == better code.
• Makes possible to reuse current time fetched elsewhere (e.g. from request).

[codecov[bot]]

Codecov Report

Merging #267 (7a66496) into master (be8e914) will not change coverage. The diff coverage is 100.00%.

:exclamation: Current head 7a66496 differs from pull request most recent head 9667b78. Consider uploading reports for the commit 9667b78 to get more accurate results

@@           Coverage Diff           @@
##           master     #267   +/-   ##
=======================================
  Coverage   92.94%   92.94%           
=======================================
  Files          15       15           
  Lines        1418     1418           
=======================================
  Hits         1318     1318           
  Misses        100      100           

Impacted Files	Coverage Δ
jose/jwt.py	98.01% <100.00%> (ø)

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