new interaction: e to escape to editor

[kohlhase]

Sometimes I do not like what I see in the file I am annotating. It would be very good if I could just type e and get an editor (via the config file which might allow configurations for multiple editors available, or as a fallback the $EDITOR variable ) with the the current file preloaded. After exiting the editor, the annotation session is resumed with the new file.

This is a simpler version of #24.

[jfschaefer]

Done