closes#4691
- adds a file handler for markdown file (.md and .markdown)
- handles titles and featured images
- gets status, date, and slug from the filename
- has a test suite
ref #4608, #4609, #4690
- fix errors with cleaning up files
- improve handling of base directories, and introduce a simple valid format for zips (must contain importable files or folders, and may contain up to one base directory)
- vastly improve test coverage
closes#4608, #4609
- image handler loads in any image files & figures out where they'll get stored
- image importer has a preprocessor which replaces image paths in
pertinent spots of post, tag and user models
- image importer stores images, keeping the path where it makes sense
- basic test for the preprocessor
