Support for subsetting fonts

[gunnsth]

Addresses #17.

Reduces test generated PDF files 2_p_nihao.pdf from 6.2MB to 45kB
2_p_multi.pdf from 287kB to 55kB

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This change is 

[codecov[bot]]

Codecov Report

Merging #335 into development will decrease coverage by 5.79%. The diff coverage is 63.30%.

@@               Coverage Diff               @@
##           development     #335      +/-   ##
===============================================
- Coverage        62.29%   56.49%   -5.80%     
===============================================
  Files              236      236              
  Lines            45607    45705      +98     
===============================================
- Hits             28412    25823    -2589     
- Misses           16542    16558      +16     
- Partials           653     3324    +2671     

Impacted Files	Coverage Δ
model/font_composite.go	61.40% <54.92%> (-8.81%)	:arrow_down:
model/font.go	57.43% <71.42%> (-6.99%)	:arrow_down:
internal/textencoding/truetype.go	40.00% <83.33%> (+20.00%)	:arrow_up:
internal/e2etest/utils.go	60.00% <0.00%> (-20.00%)	:arrow_down:
ps/exec.go	62.96% <0.00%> (-18.52%)	:arrow_down:
model/optimize/image_ppi.go	58.46% <0.00%> (-16.93%)	:arrow_down:
core/security/handlers.go	33.33% <0.00%> (-16.67%)	:arrow_down:
internal/jbig2/document/segments/region.go	63.38% <0.00%> (-15.50%)	:arrow_down:
internal/endian/endianness.go	38.46% <0.00%> (-15.39%)	:arrow_down:
model/signature_handler.go	55.85% <0.00%> (-15.32%)	:arrow_down:
... and 148 more

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[gunnsth]

@adrg Thanks for the comments. I updated slightly adding a method on the creator to mark the font for subsetting. This way is easier for the user, higher level method and does not need to worry about calling it right before writing. Can just do it right after loading.