feature: prune

[dwmkerr]

Problem

If there are icons which have not been generated (such as the round icons in issue #71), then even after running generate the expected icon might not be shown on certain devices.

Solution

Warn user when extraneous icons are present, and offer an option to have them removed, meaning only the icons created by app-icon will be in the project.

Specs

The 'prune' feature would have two parts:

Identify Extraneous Icons

When running generate, if extra icons are detected (i.e. icons which have not been generated), then a warning should be shown, such as:

$ app-icon generate.
...
Warning: Some icons in the project have not been replaced with this icon. Your icon might not be shown on all devices. Run generate with the --prune flat to remove these icons.

Prune Extra Icons

If the --prune flag is specified, any extraneous icons should be deleted.

Tasks

• [ ] Lock down technical specs
• [ ] Implement warning on generate
• [ ] Implement --prune flag on generate