all-the-icons is optional

1. [Feature] Add an option to show and hide the buffer names.
2. [Enhancement] Option doom-modeline-window-width-limit supports float number.
3. [Refactor] Use doom-modeline faces instead.
4. [Feature] Make all-the-icons optional.
5. [Feature] Support imenu and font-lock.
6. Better behaviors of doom-modeline-height and doom-modeline-width.
7. [Feature] Add Compatibility Library `compat'.
8. Optimize performance.
9. Misc bug fixing.