Add missing which-key prefix names

Unnamed Which-key prefixes:
`SPC b`, `N -> +prefix`
`SPC x`, `j -> +prefix`
have to be opened to see what kind of commands they contain.

layers/+distributions/spacemacs-base/keybindings.el

@@ -20,6 +20,7 @@
                                        ("as"  "shells")
                                        ("ay"  "ipython notebook")
                                        ("b"   "buffers")
+                                       ("bN"  "new empty buffer")
                                        ("c"   "compile/comments")
                                        ("C"   "capture/colors")
                                        ("e"   "errors")
@@ -65,6 +66,7 @@
                                        ("xa"  "align")
                                        ("xd"  "delete")
                                        ("xg"  "google-translate")
+                                       ("xj"  "justification")
                                        ("xl"  "lines")
                                        ("xm"  "move")
                                        ("xt"  "transpose")