add EncodeOptions#ignoreUndefined to ignore undefined values in maps

[gfx]

Resolve #106

[codecov-io]

Codecov Report

Merging #107 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #107      +/-   ##
==========================================
+ Coverage   98.55%   98.57%   +0.01%     
==========================================
  Files          15       15              
  Lines         902      910       +8     
  Branches      191      195       +4     
==========================================
+ Hits          889      897       +8     
  Misses         13       13

Impacted Files	Coverage Δ
src/encode.ts	100% <ø> (ø)	⬆️
src/Encoder.ts	97.92% <100%> (+0.07%)	⬆️

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