Added an output flag for saving the output to a specified file

[hellozee]

Fixes #386

[codecov-io]

Codecov Report

Merging #387 into master will decrease coverage by 0.25%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #387      +/-   ##
==========================================
- Coverage   43.37%   43.11%   -0.26%     
==========================================
  Files          15       15              
  Lines        1969     1969              
==========================================
- Hits          854      849       -5     
- Misses       1039     1043       +4     
- Partials       76       77       +1

Impacted Files	Coverage Δ
stores/flatten.go	87.28% <0%> (-4.24%)	⬇️

---

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 d2d08a1...6ce0290. Read the comment docs.

[autrilla]

Thanks for the patch! This looks good, but I think it'd be nice to have documentation for this in the README. Would you mind documenting this feature?

We might want to add an end-to-end test for this, but I don't expect you to, since those tests are written in an entirely different language :). I'll see if I have time to add it to this PR myself.

[hellozee]

@autrilla sure would love to add this flag to the documentation, can you point out to where should I add it in the readme?
If no one is onto writing a test for it, I would like to give it a try, learning a new language isn't a bad thing afterall

[autrilla]

@hellozee I'd add it under "Usage", after section 2.12 (or whatever the current latest is).

You can certainly go ahead and write a test for it!

[hellozee]

@autrilla Did I miss something?

[autrilla]

@hellozee I thought you wanted to add the functional tests for this. Let me know if you don't and I'll do it myself.

[hellozee]

@autrilla ohh I tried a couple of times to learn rust, but it didn't go too far, thought my C++ background would but nevertheless, writing tests in rust is not my thing, sadly, 😉