add support for writing custom IFDs to lazy files

[tlnagy]

fixes #105

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.06% 🎉

Comparison is base (fee7559) 90.21% compared to head (4bf3c15) 90.27%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #109      +/-   ##
==========================================
+ Coverage   90.21%   90.27%   +0.06%     
==========================================
  Files          13       13              
  Lines         756      761       +5     
==========================================
+ Hits          682      687       +5     
  Misses         74       74              

Files Changed	Coverage Δ
src/types/lazy.jl	89.85% <100.00%> (+0.79%)	⬆️

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