New feature "upload image from your computer"

Hi,

this pull request is just a proof of concept demonstrating new feature "upload image from your computer" (it is really a demo I do not expect it can be merged into your repo as it is)

It follows the idea from http://ben.onfabrik.com/posts/pagedown-markdown-editor-custom-image-dialog

It would be nice to have something like this in Galileo.

--Karel