Colorize directories, symlinks, files and the `Go up' button

Directories, symlinks, files and the `Go up' button are now
distinguishable by color.
Start the `*Augmented Tree*' buffer in `augmented-tree-mode'

The new mode causes all keybindings to show up when running
`describe-mode'.
Sort type cycling and sort order reversal toggle

- All available sort orders can now be cycled. Available are:
  `lexicographically' and `code-point'.
- Reversing the sort order works with any of the provided sort
  types and preserves it after reversal.
Indentation prefix toggling

- Indentation prefix toggling toggles a visual indentation prefix
  on/off. This allows for faster differentiation between
  indentation levels.
- Updated `README.md' and `Commentary' secion in `augmented-tree.el'.