Editor split improvements.

Added wincmd/[v]split command evaluators:

`:wincmd h/j/k/l` - move between splits.
`:wincmd[!] H/J/K/L` - move or clone (with !) editor between splits.
`:[v]split[!] [++nos[hared]]` - split (or detach with !) current editor.
  With `++nos[hared]` option put the new split outside of shared area.

Made splits contained within the shared area by default as it's behaves
more sane this way.

Added more key binding to match VIM.
Merge pull request #225 from igrekster/editor-split

Editor split improvements.