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%)	:arrow_down:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 0271651...304ee22. Read the comment docs.

[Gkreindler]

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