New variable: vimode

A separate pull request for the proposed vimode variable, see #616.
It combines the two variables ```editor_info[keymap]``` and ```editor_info[overwrite]``` into a single variable ```editor_info[vimode]```.
I recognized that this variable would be handy when I developed the budspencer, terencehill and dangerous themes. Without it, many additional if-else statements as in ```editor/init.zsh``` would be needed.