Add LimitNumTags and LimitTagSize

[bep]

With some senbible defaults.

This is a safeguard against abnormally large image metadata.

Note that there's already a Timeout option that can be set if you want to have the processing fail if it takes too long.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 85.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 76.96%. Comparing base (454e871) to head (4360618).

Files with missing lines	Patch %	Lines
imagemeta.go	88.88%	1 Missing and 1 partial ⚠️
metadecoder_iptc.go	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #39      +/-   ##
==========================================
+ Coverage   76.83%   76.96%   +0.12%     
==========================================
  Files          14       14              
  Lines        1688     1706      +18     
==========================================
+ Hits         1297     1313      +16     
- Misses        305      306       +1     
- Partials       86       87       +1     

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