Find contacts by tag

[KrashKart]

Fixes #66

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...seedu/address/logic/commands/FindByTagCommand.java	75.00%	1 Missing and 1 partial ⚠️

Files with missing lines	Coverage Δ	Complexity Δ
...du/address/logic/commands/AbstractFindCommand.java	100.00% <ø> (ø)	4.00 <0.00> (ø)
...eedu/address/logic/commands/FindByNameCommand.java	100.00% <ø> (ø)	4.00 <0.00> (ø)
.../seedu/address/logic/parser/FindCommandParser.java	82.35% <100.00%> (-1.86%)	7.00 <0.00> (+1.00)	⬇️
...ess/model/person/TagContainsKeywordsPredicate.java	100.00% <100.00%> (ø)	7.00 <7.00> (?)
...seedu/address/logic/commands/FindByTagCommand.java	75.00% <75.00%> (ø)	3.00 <3.00> (?)

[yooplo]

LGTM

This should correctly implement the finding of contacts by their tag.