Add upload file dialog

Currently, the only way to add images from local computer is via drag-and-drop - the same as github.

It would be nice to have this functionality available under the "add image" button in the editor just to make it simple for the user to figure out how gollum works with images without having to experiment.

Maybe just adding a Drag files here drop area in this dialog would be sufficient start - as same finctionality is already supported by the editor..