feat: Easy Termopen for Previewer (#88)

First pass, but seems to work. Fixed a few other related items, so pushing it to master now.

* [WIP]: Wed 16 Sep 2020 05:17:10 PM EDT

* [WIP]: Wed 16 Sep 2020 06:10:33 PM EDT

* [WIP]: Wed 16 Sep 2020 06:19:43 PM EDT

lua/telescope/actions.lua

@@ -131,6 +131,10 @@ function actions.close(prompt_bufnr)
   local prompt_win = state.get_status(prompt_bufnr).prompt_win
   local original_win_id = picker.original_win_id
 
+  if picker.previewer then
+    picker.previewer:teardown()
+  end
+
   actions.close_pum(prompt_bufnr)
   vim.cmd [[stopinsert]]