Add undo/redo

[seowalex]

No description provided.

[codecov-io]

Codecov Report

Merging #41 into master will decrease coverage by 2.23%.
The diff coverage is 24.71%.

@@             Coverage Diff              @@
##             master      #41      +/-   ##
============================================
- Coverage     60.78%   58.54%   -2.24%     
- Complexity      400      404       +4     
============================================
  Files            78       83       +5     
  Lines          1461     1544      +83     
  Branches        137      148      +11     
============================================
+ Hits            888      904      +16     
- Misses          541      606      +65     
- Partials         32       34       +2     

Impacted Files	Coverage Δ	Complexity Δ
.../java/seedu/address/logic/commands/AddCommand.java	66.66% <0.00%> (-33.34%)	8.00 <0.00> (ø)
...ain/java/seedu/address/logic/commands/Command.java	100.00% <ø> (ø)	1.00 <0.00> (ø)
...java/seedu/address/logic/commands/ExitCommand.java	100.00% <ø> (ø)	2.00 <0.00> (ø)
...java/seedu/address/logic/commands/FindCommand.java	100.00% <ø> (ø)	6.00 <0.00> (ø)
...java/seedu/address/logic/commands/HelpCommand.java	100.00% <ø> (ø)	2.00 <0.00> (ø)
...a/seedu/address/logic/commands/HistoryCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...java/seedu/address/logic/commands/ListCommand.java	100.00% <ø> (ø)	2.00 <0.00> (ø)
...java/seedu/address/logic/commands/RedoCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...java/seedu/address/logic/commands/UndoCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
.../seedu/address/logic/parser/AddressBookParser.java	80.00% <0.00%> (-20.00%)	12.00 <0.00> (ø)
... and 12 more

---

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 c283ac2...2a97567. Read the comment docs.

[zhiayang]

probably mostly fine, just reviewing cos i have to