Add `--cookie` option

[Skallwar]

Replaces #224

• downloader: add cookie support
• readme: add --cookie option

[codecov[bot]]

Codecov Report

Merging #229 (3c53005) into master (2c1f6a8) will increase coverage by 0.11%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #229      +/-   ##
==========================================
+ Coverage   80.66%   80.78%   +0.11%     
==========================================
  Files           9        9              
  Lines         331      333       +2     
==========================================
+ Hits          267      269       +2     
  Misses         64       64              

Files	Coverage Δ
src/args.rs	100.00% <ø> (ø)
src/downloader.rs	83.11% <100.00%> (+0.45%)	:arrow_up:
src/scraper.rs	87.03% <ø> (ø)