Add Undo command

[bryanyee33]

Saves entire AddressBook state before executing a modifying command.
Closes #86

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 75.62%. Comparing base (3e89032) to head (9159657).

Files	Patch %	Lines
src/main/java/seedu/address/ui/MainWindow.java	0.00%	2 Missing ⚠️
...java/seedu/address/logic/commands/CommandType.java	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #93      +/-   ##
============================================
+ Coverage     75.20%   75.62%   +0.41%     
- Complexity      413      423      +10     
============================================
  Files            68       70       +2     
  Lines          1343     1370      +27     
  Branches        110      110              
============================================
+ Hits           1010     1036      +26     
- Misses          299      300       +1     
  Partials         34       34              

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[yisiox]

LGTM