Add a wax:clobber task to remove all files generated by wax_tasks

[mnyrop]

• should remove generated pages (e.g., _qatar) and images (e.g., img/derivatives), and search index (e.g., search/index.json)
• should leave the data (e.g., _data/raw_images and _data/qatar.csv)
• BUT should remove generated columns/fields from metadata file (e.g., manifest from _data/qatar.csv)

[mnyrop]

Notes:

• feature is implemented in clobber branch but does not include tests
• does not "clobber" wax-generated metadata fields (e.g., manifest, thumbnail) because these fields might, in some cases, be manual and not wax-generated. punting this question with possibility of a wax:metadata:prune feature at a later date