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/ex-state.coffee

@@ -34,6 +34,9 @@ class ExState
   onDidFailToExecute: (fn) ->
     @emitter.on('failed-to-execute', fn)
 
+  onDidProcessOpStack: (fn) ->
+    @emitter.on('processed-op-stack', fn)
+
   pushOperations: (operations) ->
     @opStack.push operations
 
@@ -55,5 +58,6 @@ class ExState
         else
           throw e
     @clearOpStack()
+    @emitter.emit('processed-op-stack')
 
 module.exports = ExState