New feature: view current file/dir in `view-mode'

- Keybinding changes:
  + `v' is now bound to `aug-open-current-thing-in-view-mode'.
  + New key: `V' is bound to `aug-open-current-thing-read-only'
    (previously `v').
- `q' key in `view-mode-map' is overriden to jump back to
  `aug-buffer'.
- Updated `README.md'