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`)

package.json

@@ -32,5 +32,8 @@
         "0.20.0": "provideEx"
       }
     }
+  },
+  "devDependencies": {
+    "node-uuid": "^1.4.2"
   }
 }