add ranger to thread list, clean up news entries

`ranger` should've been added a while ago.  I also collapsed the various
different news items about default-not-backgroundable programs into a
single file so it will make a little more sense at the next release.

news/not_backgroundable.rst

@@ -12,11 +12,16 @@
     * cls
     * cmd
     * fish
+    * htop
     * ksh
     * less
     * man
     * more
     * mutt
+    * nano
+    * pacman
+    * ranger
+    * scp
     * sh
     * ssh
     * startx

news/scp_predict_false.rst

@@ -1,13 +0,0 @@
-**Added:** None
-
-**Changed:** None
-
-**Deprecated:** None
-
-**Removed:** None
-
-**Fixed:**
-
-* Fix ``scp`` progress not being outputted
-
-**Security:** None

news/thread_pred.rst

@@ -1,14 +0,0 @@
-**Added:**
-
-* added ``mvim``, ``pacman`` and ``nano`` to threadable predictors default-dict (all 
-  rely on ncurses)
-
-**Changed:** None
-
-**Deprecated:** None
-
-**Removed:** None
-
-**Fixed:** None
-
-**Security:** None

xonsh/commands_cache.py

@@ -266,6 +266,7 @@ def default_threadable_predictors():
         'mutt': predict_help_ver,
         'nano': predict_help_ver,
         'pacman': predict_help_ver,
+        'ranger': predict_help_ver,
         'scp': predict_false,
         'sh': predict_shell,
         'ssh': predict_false,