Implement tag find command

[Asthenosphere]

Closes #15

The following screen is the result of the command tag find 4

[sebastiantoh]

LGTM. Just checking, tagging of sales is not yet implemented right?

[codecov-io]

Codecov Report

Merging #92 into master will increase coverage by 0.71%.
The diff coverage is 70.58%.

@@             Coverage Diff              @@
##             master      #92      +/-   ##
============================================
+ Coverage     70.62%   71.33%   +0.71%     
- Complexity      760      775      +15     
============================================
  Files           124      126       +2     
  Lines          2417     2449      +32     
  Branches        289      292       +3     
============================================
+ Hits           1707     1747      +40     
+ Misses          613      602      -11     
- Partials         97      100       +3     

Impacted Files	Coverage Δ	Complexity Δ
...main/java/seedu/address/commons/core/Messages.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
...eedu/address/logic/commands/tag/DeleteCommand.java	76.47% <ø> (-1.31%)	7.00 <0.00> (ø)
.../seedu/address/logic/commands/tag/EditCommand.java	62.22% <ø> (+5.70%)	9.00 <0.00> (ø)
...du/address/logic/parser/tag/TagCommandsParser.java	37.50% <0.00%> (-5.36%)	3.00 <0.00> (ø)
src/main/java/seedu/address/model/Model.java	100.00% <ø> (ø)	2.00 <0.00> (ø)
src/main/java/seedu/address/model/AddressBook.java	60.00% <50.00%> (-0.56%)	32.00 <2.00> (+2.00)	⬇️
...rc/main/java/seedu/address/model/ModelManager.java	75.00% <50.00%> (-0.59%)	32.00 <1.00> (+1.00)	⬇️
.../seedu/address/logic/commands/tag/FindCommand.java	73.68% <73.68%> (ø)	8.00 <8.00> (?)
...du/address/logic/parser/tag/FindCommandParser.java	100.00% <100.00%> (ø)	2.00 <2.00> (?)
...tment/exceptions/AppointmentNotFoundException.java
... and 14 more

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[sebastiantoh]

LGTM!