Add tag operations for sales

[Asthenosphere]

Closes #96
Result of tag edit for sales:

Result of tag delete for sales:

Result of adding a new sale with a new tag:

[sebastiantoh]

LGTM!

[sebastiantoh]

Not really relevant to this PR, but I think the parameter and example for editing a tag is missing the updated tag name. Example should be something like tag edit 1 t/furniture. Feel free to merge this PR if you want to resolve this in another PR.

[codecov-io]

Codecov Report

Merging #96 into master will increase coverage by 0.40%.
The diff coverage is 52.29%.

@@             Coverage Diff              @@
##             master      #96      +/-   ##
============================================
+ Coverage     70.90%   71.31%   +0.40%     
- Complexity      816      837      +21     
============================================
  Files           132      132              
  Lines          2612     2698      +86     
  Branches        314      335      +21     
============================================
+ Hits           1852     1924      +72     
- Misses          649      658       +9     
- Partials        111      116       +5     

Impacted Files	Coverage Δ	Complexity Δ
...edu/address/logic/commands/contact/AddCommand.java	100.00% <ø> (ø)	8.00 <0.00> (ø)
...du/address/logic/commands/contact/EditCommand.java	95.89% <ø> (ø)	12.00 <0.00> (ø)
.../seedu/address/logic/commands/sale/AddCommand.java	30.43% <0.00%> (ø)	3.00 <0.00> (ø)
...edu/address/logic/commands/sale/DeleteCommand.java	26.31% <0.00%> (ø)	3.00 <0.00> (ø)
.../seedu/address/logic/commands/tag/FindCommand.java	77.77% <0.00%> (+4.09%)	8.00 <0.00> (ø)
src/main/java/seedu/address/model/Model.java	100.00% <ø> (ø)	2.00 <0.00> (ø)
...rc/main/java/seedu/address/model/ModelManager.java	75.51% <0.00%> (-3.22%)	36.00 <0.00> (ø)
...a/seedu/address/model/person/UniquePersonList.java	68.57% <0.00%> (-22.46%)	29.00 <0.00> (+1.00)	⬇️
src/main/java/seedu/address/model/AddressBook.java	71.79% <58.53%> (+11.11%)	47.00 <6.00> (+15.00)
src/main/java/seedu/address/model/sale/Sale.java	94.44% <87.50%> (+27.77%)	16.00 <2.00> (+2.00)
... and 13 more

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