Add tag values to CSV export

[frymash]

Tag values are now reflected alongside tag keys in the CSV export.

Resolves #139.

[codecov[bot]]

Codecov Report

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

Files with missing lines	Coverage Δ	Complexity Δ
...va/seedu/address/logic/commands/ExportCommand.java	88.73% <100.00%> (+0.49%)	17.00 <0.00> (ø)

... and 3 files with indirect coverage changes