Add Prompt for Confirmation from User

[RobotWizzard]

Adds a new field in CommandResult -- continuationFunction, that is applied when the user confirms the prompt.

MainWindow will check if the returned CommandResult requires a confirmation from user and prompts the user if it is.

Edited commands:

• ClearCommand
• DeleteClientCommand
• DeleteRentalCommand

Fixes #103

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 57.89474% with 16 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/main/java/seedu/address/ui/MainWindow.java	0.00%	16 Missing :warning:

Files with missing lines	Coverage Δ	Complexity Δ
...ava/seedu/address/logic/commands/ClearCommand.java	100.00% <100.00%> (ø)	4.00 <1.00> (+2.00)
...va/seedu/address/logic/commands/CommandResult.java	100.00% <100.00%> (ø)	21.00 <8.00> (+6.00)
...du/address/logic/commands/DeleteClientCommand.java	87.50% <100.00%> (+1.13%)	9.00 <1.00> (+2.00)
...du/address/logic/commands/DeleteRentalCommand.java	100.00% <100.00%> (ø)	13.00 <2.00> (+2.00)
src/main/java/seedu/address/ui/MainWindow.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)

... and 4 files with indirect coverage changes

[RobotWizzard]

codecov is expected to fail for MainWindow since UI components are not tested.