Add download stats

[zhuoyang125]

Create a new command download
Able to download userful statistics into a .txt file.
Update user guide to reflect this change.

TODO:

• Refactor sorting of persons
• Present stats in more user friendly way

[codecov-commenter]

Codecov Report

Merging #70 (a6419df) into master (82a0399) will decrease coverage by 4.95%.
The diff coverage is 9.21%.

@@             Coverage Diff              @@
##             master      #70      +/-   ##
============================================
- Coverage     60.78%   55.83%   -4.96%     
- Complexity      510      511       +1     
============================================
  Files           100      102       +2     
  Lines          1859     2049     +190     
  Branches        222      235      +13     
============================================
+ Hits           1130     1144      +14     
- Misses          673      848     +175     
- Partials         56       57       +1     

Impacted Files	Coverage Δ
...va/seedu/siasa/logic/commands/DownloadCommand.java	0.00% <0.00%> (ø)
.../siasa/logic/commands/policy/AddPolicyCommand.java	0.00% <0.00%> (ø)
...gic/commands/policy/ShowExpiringPolicyCommand.java	0.00% <0.00%> (ø)
...ain/java/seedu/siasa/logic/parser/SiasaParser.java	62.50% <0.00%> (-5.69%)	⬇️
...sa/logic/parser/policy/AddPolicyCommandParser.java	0.00% <0.00%> (ø)
src/main/java/seedu/siasa/model/Model.java	100.00% <ø> (ø)
src/main/java/seedu/siasa/model/ModelManager.java	83.78% <0.00%> (-2.33%)	⬇️
src/main/java/seedu/siasa/model/Siasa.java	58.06% <0.00%> (-12.53%)	⬇️
...ava/seedu/siasa/model/policy/UniquePolicyList.java	74.07% <0.00%> (-12.89%)	⬇️
src/main/java/seedu/siasa/model/tag/Tag.java	80.00% <ø> (ø)
... and 7 more

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[sherrpass]

LGTM!