feat: cycle previewers with commit and bcommit already using it (#528)

- new git previewers
- jump to line in bcommit previewer
- vimdiff for bcommits
- dynamic preview window titles
- more previewers documentation

Cycle previewers are not mapped yet. So you need to setup yourself:
```lua
require('telescope').setup {
  defaults = {
    mappings = {
      i = {
        ["<C-s>"] = actions.cycle_previewers_next,
        ["<C-a>"] = actions.cycle_previewers_prev,
      },
    },
  }
}
```

Co-authored-by: Thore Strassburg <thore@weilbier.net>

lua/telescope/previewers/previewer.lua

@@ -6,6 +6,8 @@ function Previewer:new(opts)
 
   return setmetatable({
     state = nil,
+    _title_fn = opts.title,
+    _dyn_title_fn = opts.dyn_title,
     _setup_func = opts.setup,
     _teardown_func = opts.teardown,
     _send_input = opts.send_input,
@@ -30,6 +32,20 @@ function Previewer:preview(entry, status)
   return self:preview_fn(entry, status)
 end
 
+function Previewer:title()
+  if self._title_fn then
+    return self:_title_fn()
+  end
+  return "Preview"
+end
+
+function Previewer:dynamic_title(entry)
+  if self._title_fn then
+    return self:_dyn_title_fn(entry)
+  end
+  return "Preview"
+end
+
 function Previewer:teardown()
   if self._teardown_func then
     self:_teardown_func()