Support for marked content section IDs

[dhdaines]

As requested, this is the MCID part of #937 split out. Structure tree support (using pdfminer.six) will be a separate PR.

[codecov]

Codecov Report

Merging #961 (8b5b6a3) into develop (d8b9c15) will not change coverage.
The diff coverage is 100.00%.

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##           develop      #961   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           18        18           
  Lines         1588      1613   +25     
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+ Hits          1588      1613   +25     

Files Changed	Coverage Δ
pdfplumber/page.py	100.00% <100.00%> (ø)

[dhdaines]

Note! This page only extracts marked-content identifiers for sequences of objects. There are a few other kinds is one kind of marked content that exist in PDF which it doesn't handle:

• marked-content sequences with tags (and no identifiers) - This PR will be fixed ASAP to support this DONE!
• marked-content points - These are marked points (with a tag and possible attributes) in the content stream which don't correspond to any given object. It isn't clear how this could be supported in pdfplumber.

[jsvine]

Many thanks for this, @dhdaines! It's a clever solution, and adds what seems like will be a powerful feature for people working with PDFs that have marked content.

For now, I'm going to mark mcid and tag in the README as experimental attributes, but will remove that note if/when the pdfminer.six internals that make this possible remain stable.

[dhdaines]

Many thanks for this, @dhdaines! It's a clever solution, and adds what seems like will be a powerful feature for people working with PDFs that have marked content.

For now, I'm going to mark mcid and tag in the README as experimental attributes, but will remove that note if/when the pdfminer.six internals that make this possible remain stable.

Thank you! I will submit another PR soon to add the tag attributes, as these are useful for identifying headers and footers.