Add undo command

[aidoxe-123]

• Add an undo command that undoes the previous change to the list.
  I think saving the state of the food list only when there is a change is more reasonable because if we save the state whenever a command is called, the app will look like it doesn't react if the user types in a lot of list commands, followed by a list of undo commands

• Add test cases for this feature

close #74

[codecov-io]

Codecov Report

Merging #97 into master will increase coverage by 0.34%.
The diff coverage is 95.45%.

@@             Coverage Diff              @@
##             master      #97      +/-   ##
============================================
+ Coverage     73.10%   73.45%   +0.34%     
- Complexity      463      472       +9     
============================================
  Files            78       79       +1     
  Lines          1517     1533      +16     
  Branches        122      122              
============================================
+ Hits           1109     1126      +17     
  Misses          365      365              
+ Partials         43       42       -1     

Impacted Files	Coverage Δ	Complexity Δ
.../java/jimmy/mcgymmy/logic/commands/TagCommand.java	100.00% <ø> (ø)	5.00 <0.00> (ø)
...ava/jimmy/mcgymmy/logic/commands/UnTagCommand.java	100.00% <ø> (ø)	5.00 <0.00> (ø)
src/main/java/jimmy/mcgymmy/model/Model.java	100.00% <ø> (ø)	1.00 <0.00> (ø)
...jimmy/mcgymmy/storage/JsonSerializableMcGymmy.java	100.00% <ø> (ø)	3.00 <0.00> (-1.00)
...rc/main/java/jimmy/mcgymmy/model/ModelManager.java	98.33% <92.30%> (-1.67%)	27.00 <8.00> (+4.00)	⬇️
...java/jimmy/mcgymmy/logic/commands/EditCommand.java	97.14% <100.00%> (+3.02%)	8.00 <0.00> (+2.00)
...java/jimmy/mcgymmy/logic/commands/UndoCommand.java	100.00% <100.00%> (ø)	3.00 <3.00> (?)
...y/mcgymmy/logic/parser/PrimitiveCommandParser.java	94.11% <100.00%> (+0.08%)	19.00 <0.00> (ø)
src/main/java/jimmy/mcgymmy/model/food/Food.java	97.95% <100.00%> (+2.04%)	24.00 <1.00> (+1.00)
... and 2 more

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[Jh123x]

LGTM
Might be good to add more assertions for v1.3