Add data validation

Imported data should be valid geojson. If non-geojson files are found, a warning should be displayed to the user with a list of the invalid files, but the editor should try to load all of the valid files that it found anyway.

https://github.com/cuhacking/atlas-editor/pull/13/files#diff-26ad4b834941d9b19ebf9db8082bd202aaf72ea0ddea85f5a8a0cb3c729cc6f2R47