First cut of editor split support for Juno using e4 framework.

DISCLAIMER: I had no idea what I was doing. It the first time I've touched the
e4 framework. Pure guess work.

The behaviour is different to VIM, but I believe still useful. Especially for
simple cases (two splits).

Adds the following commands:
 - `vsplit`: splits current editor vertically;
 - `split`: splits current editor horizontally;
 - `mvsplit`: moves current editor into a new vertical split;
 - `msplit`: moves current editor into a new horizontal split;
 - `wincmd_h`: switch to last active editor in the left split;
 - `wincmd_l`: switch to last active editor in the right split;
 - `wincmd_k`: switch to last active editor in the above split;
 - `wincmd_j`: switch to last active editor in the below split;
 - `wincmd_H`: move current editor into the split on the left;
 - `wincmd_L`: move current editor into the split on the right;
 - `wincmd_K`: move current editor into the split above;
 - `wincmd_J`: move current editor into the split below;
 - `wincmd_Hc`: copy/clone current editor into the split on the left;
 - `wincmd_Lc`: copy/clone current editor into the split on the right;
 - `wincmd_Kc`: copy/clone current editor into the split above;
 - `wincmd_Jc`: copy/clone current editor into the split below;