Add save for StatFiles

[TonyLianLong]

Adding save for StatFiles, #1 .

[codecov-io]

Codecov Report

Merging #16 into master will decrease coverage by 10.71%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##           master      #16       +/-   ##
===========================================
- Coverage      75%   64.28%   -10.72%     
===========================================
  Files           1        1               
  Lines          36       42        +6     
===========================================
  Hits           27       27               
- Misses          9       15        +6

Impacted Files	Coverage Δ
src/StatFiles.jl	64.28% <0%> (-10.72%)	⬇️

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[Gkreindler]

I wonder if anyone maintaining this package could look at this. It would be very useful to save to .dta for large files!