[MRG] Add warning if PDF page is image-based

[vinayak-mehta]

Closes #239.

[codecov-io]

Codecov Report

Merging #240 into master will decrease coverage by 0.07%.
The diff coverage is 86.95%.

@@            Coverage Diff             @@
##           master     #240      +/-   ##
==========================================
- Coverage   87.56%   87.49%   -0.08%     
==========================================
  Files          13       13              
  Lines        1456     1463       +7     
  Branches      338      341       +3     
==========================================
+ Hits         1275     1280       +5     
- Misses        123      124       +1     
- Partials       58       59       +1

Impacted Files	Coverage Δ
camelot/handlers.py	90.24% <100%> (ø)	⬆️
camelot/parsers/base.py	100% <100%> (ø)	⬆️
camelot/parsers/lattice.py	89.28% <100%> (+0.11%)	⬆️
camelot/parsers/stream.py	90.72% <33.33%> (-0.95%)	⬇️
camelot/utils.py	80.44% <90%> (+0.12%)	⬆️

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