add `template clean` command to remove fetched resources from template

[hapticdata]

Separating the templates from all of the resources fetched will ease the development of templates. It is convenient to be able to remove resources after they have been fetched so the user can test fetch procedures and continue to develop their templates.

[hapticdata]

Implementation plan is to generate a .fetched file that contains an Array of all files and directories added from previous fetches on the template. Running sketchplate template clean <name> would load that file and remove all items on the list, including the .fetched file.

my-template/
| .fetched
| template/
| template.json