Switch documents to unique ptr

[jeroen]

This seems to break the poppler-data hack but I don't know why.

[jeroen]

I think it happens because poppler resets the GlobalParams when the document-count goes to 0.

[codecov-io]

Codecov Report

Merging #50 into master will decrease coverage by 0.02%.
The diff coverage is 61.53%.

@@            Coverage Diff            @@
##           master     #50      +/-   ##
=========================================
- Coverage   46.92%   46.9%   -0.03%     
=========================================
  Files           4       4              
  Lines         309     307       -2     
=========================================
- Hits          145     144       -1     
+ Misses        164     163       -1

Impacted Files	Coverage Δ
src/bindings.cpp	47.52% <61.53%> (-0.03%)	⬇️

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