Misc todo before next release

[user27182]

Mix of maintenance items and features to keep track of and possibly implement before the next release. These are related to the new --doc_mode (#198), --generate_subdirs (#211 and #213), and --image_format (#216) features.

• Add a separate doc_image_format option to allow separate image formats for doc mode and regular unit tests Allow separate image format for doc mode #221
• Add a "check_cache_integrity" utility to flag any errors with files in the cache (e.g. do not allow an image named foo.png to exist along side a subdir named foo) Validate the image cache files #224
• Add option to restrict the number of allowed images in a subdir on a global basis (e.g. allow max one image per subdir), or on a per-test basis (e.g. allow exceptions to the global setting)
• Remove warning for cache subdir with one image (Remove warning for single image in subdir #234)
• Add test coverage for using subdirs in the cache with the --disallow_unused_cache option
• Allow customizing the name of image files with --doc_generate_subdirs Add hook for customizing doc mode test cases #226