Dataset recipes

With #151, FastAI.jl is getting high-level interfaces for searching datasets (`finddatasets`) and loading datasets into task-specific data containers (`loaddataset`). There is also a new `DatasetRecipe` that encapsulates configuration for loading a data container and the block information from a path. These recipes can be registered with a dataset so that they can be found using the above high-level functions.

The fastai dataset colletion comes with quite a lot of datasets, so only a few have recipes yet. This issue tracks the progress on adding recipes to all the datasets. Contributions of recipe types and recipe configs for datasets are welcome.

See `src/datasets/recipes.jl` for example recipe implementations and `src/datasets/fastairegistry` for how recipes are registered. `listdatasources()` gives you a list of all dataset sources and `datasetpath(name)` downloads them and returns the download folder.

## Progress

For datasets that can be used for multiple tasks, they are listed below. Otherwise a checked dataset that at least one recipe is already implemented.

- [x] CUB_200_2011
- [ ] bedroom (not sure how the folders are layed out)
- [x] caltech_101
- [x] cifar10
- [x] cifar100
- [ ] food-101
- [x] imagenette-160
- [x] imagenette-320
- [x] imagenette
- [x] imagenette2-160
- [x] imagenette2-320
- [x] imagenette2
- [x] imagewang-160
- [x] imagewang-320
- [x] imagewang
- [x] imagewoof-160
- [x] imagewoof-320
- [x] imagewoof
- [x] imagewoof2-160
- [x] imagewoof2-320
- [x] imagewoof2
- [x] mnist_png
- [x] mnist_var_size_tiny
- [ ] oxford-102-flowers
- [ ] oxford-iiit-pet
- [ ] stanford-cars
- [ ] ag_news_csv
- [ ] amazon_review_full_csv
- [ ] amazon_review_polarity_csv
- [ ] dbpedia_csv
- [ ] giga-fren
- [ ] imdb
- [ ] sogou_news_csv
- [ ] wikitext-103
- [ ] wikitext-2
- [ ] yahoo_answers_csv
- [ ] yelp_review_full_csv
- [ ] yelp_review_polarity_csv
- [ ] biwi_head_pose
- [x] camvid
- [ ] pascal-voc
- [ ] pascal_2007
- [x] multi-label image classification (`(Image{2}, LabelMulti)`)
- [ ] object detection
- [ ] pascal_2012
- [ ] siim_small
- [ ] skin-lesion
- [ ] tcga-small
- [x] adult_sample
- [ ] biwi_sample
- [x] camvid_tiny
- [ ] dogscats
- [ ] human_numbers
- [ ] imdb_sample
- [x] mnist_sample
- [x] mnist_tiny
- [ ] movie_lens_sample
- [ ] planet_sample
- [ ] planet_tiny
- [ ] coco_sample
- [ ] coco-train2017
- [ ] coco-val2017
- [ ] coco-test2017
- [ ] coco-unlabeled2017
- [ ] coco-image_info_test2017
- [ ] coco-image_info_unlabeled2017
- [ ] coco-annotations_trainval2017
- [ ] coco-stuff_annotations_trainval2017
- [ ] coco-panoptic_annotations_trainval2017