Add specs; minor changes to some commands

`:tabedit` now works as an alias to `:edit` with a path and
  as an alias to `:tabnew` without.
`:tabnew` is a new command that opens a new tab with a new file if used
 without a path and works as an alias to `:tabedit` with one.
`:tabclose` now works as a proper alias to `:quit` (i.e. passes the arguments)
`:edit` now works more like before - it opens a given path in a new tab.
 It also doesn't do anything if the file was modified since the last commit,
 unless forced by using `:edit!`
`:write` works properly again and doesn't overwrite files, unless forced by
 using `:write!`
`:xit` is now called `:xit` and not just `:x`
`:substitute` now properly replaces multiple groups (`:s/(a)b(c)/X\1\2X\0`)

lib/view-model.coffee

@@ -1,11 +1,11 @@
-ExCommandModeInputElement = require './ex-command-mode-input-element'
+ExNormalModeInputElement = require './ex-normal-mode-input-element'
 
 class ViewModel
   constructor: (@command, opts={}) ->
     {@editor, @exState} = @command
 
-    @view = new ExCommandModeInputElement().initialize(@, opts)
-    @editor.commandModeInputView = @view
+    @view = new ExNormalModeInputElement().initialize(@, opts)
+    @editor.normalModeInputView = @view
     @exState.onDidFailToExecute => @view.remove()
     @done = false