Add snap command

[aizatazhar]

Closes #121

snap command creates a snapshot of the current address book and saves it as a json file with the user specified file name

[codecov]

Codecov Report

Merging #124 into master will decrease coverage by 0.42%.
The diff coverage is 57.50%.

@@             Coverage Diff              @@
##             master     #124      +/-   ##
============================================
- Coverage     71.38%   70.96%   -0.43%     
- Complexity      423      434      +11     
============================================
  Files            74       76       +2     
  Lines          1342     1381      +39     
  Branches        133      137       +4     
============================================
+ Hits            958      980      +22     
- Misses          343      359      +16     
- Partials         41       42       +1     

Impacted Files	Coverage Δ	Complexity Δ
...va/seedu/address/logic/commands/DeleteCommand.java	100.00% <ø> (ø)	9.00 <0.00> (ø)
.../seedu/address/logic/parser/ZooKeepBookParser.java	84.21% <0.00%> (-4.68%)	12.00 <0.00> (ø)
src/main/java/seedu/address/model/Model.java	100.00% <ø> (ø)	1.00 <0.00> (ø)
src/main/java/seedu/address/model/ZooKeepBook.java	91.66% <ø> (ø)	12.00 <0.00> (ø)
...c/main/java/seedu/address/model/animal/Animal.java	97.50% <ø> (ø)	18.00 <0.00> (ø)
src/main/java/seedu/address/model/animal/Name.java	80.00% <ø> (ø)	6.00 <0.00> (ø)
...a/seedu/address/model/animal/UniqueAnimalList.java	92.68% <ø> (ø)	20.00 <0.00> (ø)
...java/seedu/address/logic/commands/SnapCommand.java	51.72% <51.72%> (ø)	6.00 <6.00> (?)
.../seedu/address/logic/parser/SnapCommandParser.java	77.77% <77.77%> (ø)	5.00 <5.00> (?)
.../java/seedu/address/logic/commands/AddCommand.java	100.00% <100.00%> (ø)	8.00 <1.00> (ø)
... and 2 more

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