Various previewer fixes (#260)

- Set env vars for previewer with set_env({ key = value } fix #228)
- Set pager for git previewes (default is GIT_PAGER=less so users can scroll see #253)
- Use less for people who don't have bat but less installed
- Reset less env to only '-RS' because '-X' will break scrolling(fix #253, thanks @eth3lbert)
- Scrolling for buffer previewer(thanks @tami5 for helping)
  - Also new function to create buffer previewers (which has scrolling by default). Used for tags and help_tags previewer.
- Add new setting set_env to readme

lua/telescope/config.lua

@@ -62,10 +62,12 @@ function config.set_defaults(defaults)
   set("use_less", true)
   set("color_devicons", true)
 
+  set("set_env", {})
+
   -- TODO: Add motions to keybindings
 
   -- To disable a keymap, put [map] = false
-  --        So, to not map "<C-n>", just put 
+  --        So, to not map "<C-n>", just put
   --
   --            ...,
   --            ["<C-n>"] = false,