Add an option to parse token from POST form data.

[Starballoon]

As mentioned in Issue 292, we all participated in the training camp organized by ByteDance. They recommended this lightweight middleware for our project. The job assigned to us needs to parse the token from the POST form data, which have not been supported by this middleware. So I wish to add a parsing option to support this and it works fine in my own project.

[codecov-commenter]

Codecov Report

Merging #293 (b4eaf1c) into master (687525f) will decrease coverage by 1.81%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #293      +/-   ##
==========================================
- Coverage   90.45%   88.64%   -1.82%     
==========================================
  Files           1        1              
  Lines         440      449       +9     
==========================================
  Hits          398      398              
- Misses         30       39       +9     
  Partials       12       12              

Flag	Coverage Δ
go-1.16	88.64% <0.00%> (-1.82%)	⬇️
go-1.17	88.64% <0.00%> (-1.82%)	⬇️
macos-latest	88.64% <0.00%> (-1.82%)	⬇️
ubuntu-latest	88.64% <0.00%> (-1.82%)	⬇️

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Impacted Files	Coverage Δ
auth_jwt.go	88.64% <0.00%> (-1.82%)	⬇️

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