Change Tag to be case insensitive

[DesSnowy]

closes #227

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Files with missing lines	Coverage Δ	Complexity Δ
src/main/java/seedu/address/model/tag/Tag.java	71.42% <100.00%> (ø)	5.00 <1.00> (ø)