Refactor the installer to be more maintainable and interactive. The installer is now more robust and safe in its operation, and asks interactive questions for choices that the user must resolve. Setting up confuguration is also now changed to take advantage of Fish 2.3 features and does not mess with user's configuration files without permission.

- Make installer smarter and use uninstaller in destroy
- Better handling and checking for offline installs
- `omf destroy` simply uses `install --uninstall`
- Final warning message is no longer displayed when user intentionally aborts install
- Backups are restored during uninstall
- Update the README to detail the new ways to install OMF