feat(lib): add loop option to next_slide and remove start/end_loop (#294)

* feat(lib): add `loop` option to `next_slide` and remove `start/end_loop`

* fix(docs): PR number

docs/source/reference/api.md

@@ -14,11 +14,9 @@ use, not the methods used internally when rendering.
         add_to_canvas,
         canvas,
         canvas_mobjects,
-        end_loop,
         mobjects_without_canvas,
         next_slide,
         remove_from_canvas,
-        start_loop,
         wait_time_between_slides,
         wipe,
         zoom,

docs/source/reference/magic_example.ipynb

@@ -58,10 +58,9 @@
     "        ).arrange(DOWN, buff=1.)\n",
     "        \n",
     "        self.play(Write(text))\n",
-    "        self.next_slide()\n",
-    "        self.start_loop()\n",
+    "        self.next_slide(loop=True)\n",
     "        self.play(Indicate(text[-1], scale_factor=2., run_time=.5))\n",
-    "        self.end_loop()\n",
+    "        self.next_slide()\n",
     "        self.play(FadeOut(text))"
    ]
   },