Add support for custom task labels

[mcaskill]

If ever the basename from the outfile or outdir is an insufficient description.

For the boilerplate's currently available tasks, this feature is not very important but could be for future tasks. It at least serves as inspiration for custom tasks for specific projects. For example, in two client projects I have a custom tasks/copies.js that benefits from a custom label:

Example 1

```json "copies": [ { "label": "flatpickr", "src": "./node_modules/flatpickr/dist", "dest": "./www/assets/admin/flatpickr" } ] ```

Example 2

```json "concats": [ { "label": "third-parties", "includes": [ "{% paths.scripts.src %}/vendors/*.js" ], "outfile": "{% paths.scripts.dest %}/vendors.js" } ] ```